Update spectrum class name from Spectrum1D to Spectrum

[rosteen]

Does what it says on the tin - based on the result of #1087. Spectrum1D will raise a deprecation warning when initialized for some deprecation period.

[codecov]

Codecov Report

Attention: Patch coverage is 75.80645% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 72.72%. Comparing base (4041081) to head (40ac61b).
Report is 37 commits behind head on v2.0-dev.

Files	Patch %	Lines
specutils/io/default_loaders/apogee.py	25.00%	3 Missing ⚠️
specutils/io/default_loaders/sdss.py	25.00%	3 Missing ⚠️
specutils/io/default_loaders/sixdfgs_reader.py	25.00%	3 Missing ⚠️
specutils/spectra/spectrum.py	75.00%	3 Missing ⚠️
specutils/io/default_loaders/aaomega_2df.py	33.33%	2 Missing ⚠️
specutils/io/default_loaders/twodfgrs_reader.py	33.33%	2 Missing ⚠️
specutils/io/default_loaders/wcs_fits.py	71.42%	2 Missing ⚠️
specutils/manipulation/smoothing.py	60.00%	2 Missing ⚠️
specutils/manipulation/utils.py	75.00%	2 Missing ⚠️
specutils/io/default_loaders/dc_common.py	50.00%	1 Missing ⚠️
... and 7 more

Additional details and impacted files

@@             Coverage Diff              @@
##           v2.0-dev    #1126      +/-   ##
============================================
+ Coverage     71.26%   72.72%   +1.45%     
============================================
  Files            64       62       -2     
  Lines          4542     4487      -55     
============================================
+ Hits           3237     3263      +26     
+ Misses         1305     1224      -81     

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[rosteen]

Looks like the test failure is unrelated, I wonder if that MANGA file was removed or changed url or something...

[rosteen]

This is probably the most important part if anyone reviews:

@deprecated(since="2.0", alternative="Spectrum")
class Spectrum1D(Spectrum):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)