ENH: add comparison operators to exprValue, integrate exprValueFieldTag

- exprValueFieldTag is an extended version of exprValue,
  with additional Field/List uniformity handling

- the exprValueFieldTag reduce() method provides a more efficient
  method than using a regular combine operator. Since fields are
  usually non-uniform, will mostly only need the bitwise reduce and
  not a more expensive gather/combine.

ENH: output of exprValue (scalar type) now includes '.'

- prevents scalar/label ambiguity for values like '100.0', which would
  otherwise be written as '100' and thus interpreted as a label value
  when re-reading.