* Support default values for format/compress enum lookups.

  - Avoids situations where the preferred default format is not ASCII.
    For example, with dictionary input:

        format binar;

    The typing mistake would previously have caused formatEnum to
    default to ASCII. We can now properly control its behaviour.

        IOstream::formatEnum
        (
            dict.get<word>("format"), IOstream::BINARY
        );

    Allowing us to switch ascii/binary, using BINARY by default even in
    the case of spelling mistakes. The mistakes are flagged, but the
    return value can be non-ASCII.

* The format/compression lookup behave as pass-through if the lookup
  string is empty.

  - Allows the following to work without complaint

      IOstream::formatEnum
      (
          dict.getOrDefault("format", word::null), IOstream::BINARY
      );

  - Or use constructor-like failsafe method

      IOstream::formatEnum("format", dict, IOstream::BINARY);

  - Apply the same behaviour with setting stream format/compression
    from a word.

       is.format("binar");

    will emit a warning, but leave the stream format UNCHANGED

* Rationalize versionNumber construction

  - constexpr constructors where possible.
    Default construct is the "currentVersion"

  - Construct from token to shift the burden to versionNumber.
    Support token as argument to version().

    Now:

        is.version(headerDict.get<token>("version"));

    or failsafe constructor method

        is.version
        (
            IOstreamOption::versionNumber("version", headerDict)
        );

    Before (controlled input):

        is.version
        (
            IOstreamOption::versionNumber
            (
                headerDict.get<float>("version")
            )
        );

    Old, uncontrolled input - has been removed:

        is.version(headerDict.lookup("version"));

* improve consistency, default behaviour for IOstreamOption construct

  - constexpr constructors where possible

  - add copy construct with change of format.

  - construct IOstreamOption from streamFormat is now non-explicit.
    This is a commonly expected result with no ill-effects