functionObjects, foamCalcFunctions, fvOptions: Standardized keywords for...

functionObjects, foamCalcFunctions, fvOptions: Standardized keywords for selecting fields and objects

Generally fields and objects are selected using the 'field[s]' and
'object[s]' keywords but this was not consistent between all
functionObject, fvOptions etc. and now fixed by applying the following
renaming:

fieldName -> field
fieldNames -> fields
objectName -> object
objectNames -> objects