Update lightcurve.py

[Pablogh292]

Fixes an issue regarding lightcurve.py crashing when a time bin has NaN values. Generally speaking all the code does is add an extra try/except block to handle the RuntimeError thrown out by gtanalysis.py when it encounters NaN values.

[codecov]

Codecov Report

Attention: Patch coverage is 43.37349% with 47 lines in your changes are missing coverage. Please review.

Project coverage is 49.21%. Comparing base (fc0801a) to head (a967ef4).

Files	Patch %	Lines
fermipy/lightcurve.py	43.37%	47 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #587      +/-   ##
==========================================
- Coverage   49.29%   49.21%   -0.08%     
==========================================
  Files         143      143              
  Lines       26962    27005      +43     
==========================================
+ Hits        13290    13291       +1     
- Misses      13672    13714      +42     

Flag	Coverage Δ
unittests	49.21% <43.37%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[omodei]

Is it possible to isolate the problem a bit more, instead of incorporating so many lines of code in the trt/except block? Also, I would suggest to except with catching the exception.