ENH: patch fields writeValueEntry method (frequently used)

- add base-level readValueEntry, readMixedEntries methods
  that allow optional or mandatory reading.

STYLE: wording for patch fields comments

etc/codeTemplates/BC/BC.C

@@ -225,7 +225,7 @@ void Foam::CLASS::write
     fieldData_.writeEntry("fieldData", os);
     timeVsData_->writeData(os);
     os.writeEntry("wordData", wordData_);
-    this->writeEntry("value", os);
+    FVPATCHF::writeValueEntry(os);
 }