Simplify TimeParser.

To simplify the codebase a bit, TimeParser is now:

* No longer constructed with a customisable unit map; the units are mapped to multipliers by an internal `if` statement.
* No longer a template parameterised over the types involved; the output type is now fixed to an alias of `std::uint64_t`.

This pull request also contains some unit tests for checking that TimeParser handles _valid_ time-strings.  This is not a complete test suite, as it does not check exceptions.  Currently, I'm unsure how to use Catch for exception tests, so we only check the 'valid' cases.
Oh, _that_.  I see your point; IIRC in the BAPS3 spec I only said that the short form was required, so the long form is probably not necessary.  I kept it in mainly for backwards-compatibility with the old code but if everyone's agreed we can remove it.