Introduce gammapy.datasets submodule

[adonath]

Description

This pull request introduce a new gammapy.datasets sub-module. The motivation for this is outlined in PIG-19 (see #2720). It includes the following changes:

• Move cube/fit.py to datasets/map.py
• Move spectrum/dataset.py to datasets/spectrum.py
• Move spectrum/flux_points.py to datasets/flux_points.py
• Move the dataset part of modelling/serialize.py to datasets/io.py
• Move DATASETS registry to gammapy/datasets
• Introduce gammapy/utils/registry.py
• Preliminarily Move CountsSpectrum to gammapy.maps
• Remove unused simulate_dataset function

Dear reviewer
No need for a real review. I'll wait for Travis to be green and then merge.

[codecov]

Codecov Report

Merging #2788 into master will increase coverage by 0.06%.
The diff coverage is 92.92%.

@@            Coverage Diff             @@
##           master    #2788      +/-   ##
==========================================
+ Coverage   92.21%   92.27%   +0.06%     
==========================================
  Files         144      148       +4     
  Lines       16576    16576              
==========================================
+ Hits        15286    15296      +10     
+ Misses       1290     1280      -10

Impacted Files	Coverage Δ
gammapy/cube/__init__.py	100% <ø> (ø)	⬆️
gammapy/modeling/__init__.py	100% <ø> (ø)	⬆️
gammapy/maps/counts_spectrum.py	93.18% <ø> (ø)
gammapy/spectrum/__init__.py	100% <ø> (ø)	⬆️
gammapy/datasets/map.py	91.02% <100%> (ø)
gammapy/modeling/fit.py	93.52% <100%> (ø)	⬆️
gammapy/time/lightcurve_estimator.py	96.12% <100%> (+0.02%)	⬆️
gammapy/modeling/models/__init__.py	100% <100%> (+21.42%)	⬆️
gammapy/maps/wcsnd.py	96.36% <100%> (ø)	⬆️
gammapy/data/event_list.py	90.11% <100%> (ø)	⬆️
... and 26 more

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