ENH: make precision adaptors modifiable (#2173)

- allows reuse similar to refPtr for wrapping different content.

- additional control for when contents are copied back,
  instead of waiting for the adaptor to go out of scope.

  Eg,

  if (adaptor.active())
  {
      adaptor.commit();
      adaptor.clear();
  }

- static ConstPrecisionAdaptor::get method renamed to 'select' as a
  better description of its purpose and avoid confusion with
  non-static 'get' method.

  Was previously only used within GAMGPreconditioner, but even there
  it is better just to use the ConstPrecisionAdaptor directly.

applications/test/PrecisionAdaptor/Make/files

@@ -0,0 +1,3 @@
+Test-PrecisionAdaptor.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-PrecisionAdaptor

applications/test/PrecisionAdaptor/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */

applications/test/PrecisionAdaptor/Test-PrecisionAdaptor.C

@@ -0,0 +1,111 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2021 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-PrecisionAdaptor
+
+Description
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "primitiveFields.H"
+#include "PrecisionAdaptor.H"
+
+using namespace Foam;
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    Field<double> content1(8);
+    Field<double> content2(8);
+
+    forAll(content1, i)
+    {
+        content1[i] = 10 * i;
+        content2[i] = 10 * i;
+    }
+
+    Foam::reverse(content2);
+
+    ConstPrecisionAdaptor<float, double, Field> cadaptor1;
+
+    cadaptor1.set(content1);
+    cadaptor1.commit();  // This is a no-op
+    Info<< "wrapped: " << cadaptor1() << nl;
+
+    cadaptor1.set(content2);
+    Info<< "wrapped: " << cadaptor1() << nl;
+
+    Info<< nl;
+
+    PrecisionAdaptor<float, double, Field> adaptor2;
+
+    adaptor2.set(content1);
+    adaptor2.ref() *= 2;
+    adaptor2.commit();  // Propagate changes back to input now
+
+    Info<< "modified wrapped: " << adaptor2() << nl;
+
+    adaptor2.set(content2);
+    adaptor2.ref() *= 2;
+    adaptor2.commit();  // Propagate changes back to input now
+
+    Info<< "modified wrapped: " << adaptor2() << nl;
+    Info<< "source: " << content1 << nl;
+    Info<< "source: " << content2 << nl;
+
+
+    content2 *= 2;
+    Info<< nl
+        << "set with " << content2 << nl;
+    Info<< "wrapped was " << adaptor2() << nl;
+    adaptor2.set(content2);
+    Info<< "wrapped now " << adaptor2() << nl;
+    Info<< "source: " << content2 << nl;
+
+    // Can even do this
+    Foam::reverse(adaptor2.ref());
+
+    adaptor2.ref() *= 2;
+    adaptor2.set(content1);  // implicit commit
+    Info<< "updated: " << content2 << nl;
+
+    Info<< nl
+        << "input: " << content1 << nl;
+
+    adaptor2.ref() *= 2;
+    adaptor2.clear();  // discard
+    adaptor2.commit();
+
+    Info<< "unchanged: " << content1 << nl;
+
+    Info<< nl << "Done" << nl << endl;
+    return 0;
+}
+
+
+// ************************************************************************* //