Add invalid input parameter exceptions to add_flare_signal

[scfleming]

To support direct unit testing and use of the function outside of the main sim_lc.py, add catch-and-raise-exceptions for the following:

1. a negative value for flare_amp is submitted
2. a negative value for flare_halfwidth is submitted
3. a flare_halfwidth of 0 is submitted

and also update the unit test file for these cases