Allow inverse (negative timestep) or instantaneous (0 timestep) models

[dorchard]

This PR allows two new classes of model:

• those which are temporally 'inverse', i.e., where the end time is before the start time and the time step is negative
• those which are 'instantaneous', i.e., the time step is 0 and the start and end time as the same (otherwise there is an infinite number of steps).

This PR generalises the tests that (previously) checked that the current time is <= the start and <= the end (now it can be the opposite if the time step is negative, or it can be the same if the time step is 0- this is redundandtly broken into its own case but for clarity). Similarly it generalises the tests checking the relationship between the start and end.

This has been tested on pymt_hydrotrend.

(see discussion here: https://github.com/orgs/csdms/discussions/26)