Fix nsteps bug

[corakingdon]

This implements the following, which is my interpretation of the two use cases of the nsteps option to get_model:

1. If a user wants to use the nsteps option just to shorten the time dimension, still using all the default FUND parameter values, then we need to wait to reset the time dimension until after calling set_leftover_params!, since this currently produces an error from Mimi since the default parameter values are longer than the requested time dimension.
2. But if a user wants nsteps to be longer than the default_nsteps, then they need to provide an alternative datadir or params dictionary with values that will work for that length of time dimension. In this case, we set the new time dimension before setting the parameter values.

[corakingdon]

This will currently fail because of this test on runtests.jl line 28:

new_nsteps = 10
@test_throws ErrorException m2 = MimiFUND.get_model(nsteps = new_nsteps) #should error because parameter lenghts won't match time dim

But if we decide we want to allow this behavior, then I will delete this test and add others for the new functionality