fix input for background.traces, raise error in FlatTrace for negative trace

[cshanahan1]

This PR cleans up a few things in Background.

1. Added a condition to FlatTrace that trace position must be negative. This check was originally done in Background when a FlatTrace was being used, but should generally be done. Before, if trace_pos was negative, a fully masked trace was being returned.

2. Previously, Background produced an unrelated error if 'traces' was None. Now, if no trace/number is passed in, a FlatTrace in the middle of the image will be used.

3. There was some other inconsistencies with inputs - the docstring said 'traces' must be a list, but the logic in the code implied that it could be a Trace object not in a list. Also, a FlatTrace object can be defined with either a float or an int, where previously 'Background' allowed only integer inputs. I fixed this to allow either single trace/int/float or list trace/int/float inputs. The only condition is that lists must all be of the same type (either numeric or Trace). I also added some checks for input to make sure something valid is passed in for traces (either None, int/float or list of int/float to define FlatTraces, or a Trace object, or a list of Trace objects). I also added tests for all of these allowed inputs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.16%. Comparing base (85e1b20) to head (ddc7aab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
+ Coverage   81.94%   82.16%   +0.22%     
==========================================
  Files          10       10              
  Lines        1008     1015       +7     
==========================================
+ Hits          826      834       +8     
+ Misses        182      181       -1     

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[tepickering]

the changes look good to me. thanks for the cleanup and fixes. however, something about the changes caused the RTD build to fail. i looked at the raw build logs and found the warnings that point to tracing.py, but don't quite understand why it's not happy.

[cshanahan1]

fixed the failing docstring

[tepickering]

thanks for the quick response. all the checks are happy so i am as well. merge away!