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.
good points, but equally `Time` gives no indication of what it actually is. "Is it an int, a struct, or a function pointer? I don't know, now I've got to go and look it up"

And is it ever likely to change? 2**32-1 microseconds is 72 minutes, which is probably not large enough to be sure of not-overflows