Custom invalid value handling for integer

Closes DaVinci#58 (closed) and Rec#270 (closed)

• Remove InvalidValue<int> and force users to specify default custom value for integers.
• If variables are not specified for a given field remove that field automatically rather than throwing an error.
• Some clean up of functorcollections.

To be tested with: Rec!2992 (merged), DaVinci!720 (merged)

cc: @chasse, @graven, @erodrigu