Using prefix keyword in psmap.

[ndilalla]

This PR solves issue #643. List of changes:

1. Use prefix in the map name:

• o['name'] = utils.join_strings([prefix, 'PSmap']) so with prefix='state1' the name is state1_PSmap.
• with prefix='' it stays PSmap.

2. Use prefix for the default FITS path:

• With no outfile, path is utils.format_filename(self.workdir, 'PSmap', prefix=[prefix]), so e.g. state1_PSmap.fits when a prefix is set.

3. Stopped overwriting o['name'] with outfile:

• o['name'] is always utils.join_strings([prefix, 'PSmap']) so that plot filenames always respect prefix.
• Only the FITS path is taken from outfile when it’s provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 50.79%. Comparing base (92cf94f) to head (5da6366).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
fermipy/psmap.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #660   +/-   ##
=======================================
  Coverage   50.79%   50.79%           
=======================================
  Files         143      143           
  Lines       27265    27263    -2     
=======================================
  Hits        13849    13849           
+ Misses      13416    13414    -2     

Flag	Coverage Δ
unittests	50.79% <91.66%> (+<0.01%)	⬆️

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