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.
Yes, in [Player][].  (The reason this throws `std::out_of_range` is because it replaces a `map.at()`, so I kept the exception from that.)

[Player]: https://github.com/UniversityRadioYork/ury-playd/blob/ea041e3af0ef5628215e933e5e9ebc59e323fd90/src/player/player.cpp#L151-L154