Update to handle DR4 catalog

[facero]

Hi,

This PR implements the DR4 catalog in the existing framework.
The PR adds the catalog gll_psc_v35 + new extended source archive (Extended_14years) + updates to the catalog.py. There are no new difference in the catalog format so this is pretty straightforward.
This PR fixes the issues raised in #607 and #632 .

It works locally.
@ndilalla tests are welcome !

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 52 lines in your changes missing coverage. Please review.

Project coverage is 50.58%. Comparing base (4ccc314) to head (d4f5da5).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
fermipy/catalog.py	7.14%	52 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
- Coverage   50.67%   50.58%   -0.09%     
==========================================
  Files         143      143              
  Lines       27196    27252      +56     
==========================================
+ Hits        13782    13786       +4     
- Misses      13414    13466      +52     

Flag	Coverage Δ
unittests	50.58% <7.14%> (-0.09%)	⬇️

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[ndilalla]

Thanks @facero! I will merge this PR to the master and add a test for 4FGL-DR4