ENH: improve handling of finiteArea mesh with distributed roots

- in makeFaMesh, the serial fields are now only read on the master
  process and broadcast to the other ranks. The read+distribute is
  almost identical to that used in redistributePar, except that in
  this case entire fields are sent and not a zero-sized subset.

- improved internal faMesh checking for files so that the TryNew
  method works with distributed roots.

src/parallel/decompose/faDecompose/faFieldDecomposer.H

@@ -45,6 +45,7 @@ SourceFiles
 #define Foam_faFieldDecomposer_H
 
 #include "faMesh.H"
+#include "faMeshSubset.H"
 #include "faPatchFieldMapper.H"
 #include "edgeFields.H"
 
@@ -446,6 +447,26 @@ public:
             const IOobjectList& objects
         );
 
+        //- Read all fields given mesh and objects.
+        //- Supports reading/sending fields
+        void readAllFields
+        (
+            const bitSet& haveMeshOnProc,
+            const faMeshSubset* subsetter,
+            const faMesh& mesh,
+            IOobjectList& objects
+        );
+
+        //- Read all fields given mesh and objects.
+        //- Supports reading/sending fields
+        void readAllFields
+        (
+            const boolList& haveMeshOnProc,
+            const faMeshSubset* subsetter,
+            const faMesh& mesh,
+            IOobjectList& objects
+        );
+
         //- Decompose and write all fields
         void decomposeAllFields
         (

src/parallel/decompose/faDecompose/faFieldDecomposerCache.C

@@ -77,7 +77,11 @@ public:
 
     bool empty() const noexcept { return !size(); }
 
-    void readAll(const faMesh& mesh, const IOobjectList& objects)
+    void readAll
+    (
+        const faMesh& mesh,
+        const IOobjectList& objects
+    )
     {
         #undef  doLocalCode
         #define doLocalCode(Type)                                     \
@@ -113,6 +117,45 @@ public:
         #undef doLocalCode
     }
 
+    template<class BoolListType>
+    void readAll
+    (
+        const BoolListType& haveMeshOnProc,
+        const faMeshSubset*& subsetter,
+        const faMesh& mesh,
+        IOobjectList& objects
+    )
+    {
+        #undef  doLocalCode
+        #define doLocalCode(Type)                                     \
+        {                                                             \
+            fieldsDistributor::readFields                             \
+            (                                                         \
+                haveMeshOnProc,                                       \
+                subsetter,                                            \
+                mesh,                                                 \
+                objects,                                              \
+                Type##AreaFields_                                     \
+            );                                                        \
+            fieldsDistributor::readFields                             \
+            (                                                         \
+                haveMeshOnProc,                                       \
+                subsetter,                                            \
+                mesh,                                                 \
+                objects,                                              \
+                Type##EdgeFields_                                     \
+            );                                                        \
+        }
+
+        doLocalCode(scalar);
+        doLocalCode(vector);
+        doLocalCode(sphericalTensor);
+        doLocalCode(symmTensor);
+        doLocalCode(tensor);
+
+        #undef doLocalCode
+    }
+
     template<class GeoField>
     static void decompose
     (
@@ -205,6 +248,36 @@ void Foam::faFieldDecomposer::fieldsCache::readAllFields
 }
 
 
+void Foam::faFieldDecomposer::fieldsCache::readAllFields
+(
+    const bitSet& haveMeshOnProc,
+    const faMeshSubset* subsetter,
+    const faMesh& mesh,
+    IOobjectList& objects
+)
+{
+    if (cache_)
+    {
+        cache_->readAll(haveMeshOnProc, subsetter, mesh, objects);
+    }
+}
+
+
+void Foam::faFieldDecomposer::fieldsCache::readAllFields
+(
+    const boolList& haveMeshOnProc,
+    const faMeshSubset* subsetter,
+    const faMesh& mesh,
+    IOobjectList& objects
+)
+{
+    if (cache_)
+    {
+        cache_->readAll(haveMeshOnProc, subsetter, mesh, objects);
+    }
+}
+
+
 void Foam::faFieldDecomposer::fieldsCache::decomposeAllFields
 (
     const faFieldDecomposer& decomposer,