ENH: HashTable cfind() method returning a const_iterator

- This follows the same idea as cbegin/cend and is helpful when using
  C++11 auto to ensure we have unambiguous const-safe access.

  Previously:
  ====
    typename someLongClass::const_iterator iter = someTable.find(key);

    ... later on:
    *iter = value; // Oops, but caught by compiler.

  We can save some typing with auto, but it is uncertain what we get:
  ====
    auto iter = someTable.find(key);
        // iterator or const_iterator?
        // depends on someTable having const or non-const access.

    ... later on:
    *iter = value;  // Oops, but not caught by compiler.

  Using cfind instead, auto will deduce const_iterator as the type:
  ====
    auto iter = someTable.cfind(key);  // definitely const_iterator

    ... later on:
    *iter = value; // Oops, but caught by compiler.

src/OpenFOAM/containers/HashTables/HashTable/HashTable.C

@@ -74,7 +74,7 @@ Foam::HashTable<T, Key, Hash>::HashTable(const label size)
     {
         table_ = new hashedEntry*[tableSize_];
 
-        for (label hashIdx = 0; hashIdx < tableSize_; hashIdx++)
+        for (label hashIdx = 0; hashIdx < tableSize_; ++hashIdx)
         {
             table_[hashIdx] = nullptr;
         }
@@ -203,6 +203,17 @@ Foam::HashTable<T, Key, Hash>::find
 (
     const Key& key
 ) const
+{
+    return this->cfind(key);
+}
+
+
+template<class T, class Key, class Hash>
+typename Foam::HashTable<T, Key, Hash>::const_iterator
+Foam::HashTable<T, Key, Hash>::cfind
+(
+    const Key& key
+) const
 {
     if (nElmts_)
     {
@@ -912,7 +923,7 @@ bool Foam::HashTable<T, Key, Hash>::operator==
 
     for (const_iterator iter = rhs.cbegin(); iter != rhs.cend(); ++iter)
     {
-        const_iterator other = find(iter.key());
+        const_iterator other = this->cfind(iter.key());
 
         if (!other.found() || other.object() != iter.object())
         {