Implement FluxEstimator

[registerrier]

Description

This pull request introduces the FluxEstimator.
So far result return is in the form of a dict. Do we prefer to have a Table row instead?
Dear reviewer

Some work still needed.
The early PR should help test with the new scipy 1.4.1.

[codecov]

Codecov Report

Merging #2810 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2810      +/-   ##
==========================================
+ Coverage   92.17%   92.21%   +0.03%     
==========================================
  Files         147      148       +1     
  Lines       16636    16678      +42     
==========================================
+ Hits        15334    15379      +45     
+ Misses       1302     1299       -3

Impacted Files	Coverage Δ
gammapy/estimators/parameter_estimator.py	87.05% <ø> (-0.16%)	⬇️
gammapy/estimators/__init__.py	100% <100%> (ø)	⬆️
gammapy/estimators/flux.py	100% <100%> (ø)
gammapy/modeling/models/cube.py	92.3% <0%> (+0.61%)	⬆️
gammapy/datasets/core.py	87.05% <0%> (+0.71%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 277ede6...d359c08. Read the comment docs.

[adonath]

Thanks @registerrier, I've left a few inline comments.

Minor: I think the _estimator suffix in the file name is not needed, as the file is already in the gammapy.estimators sub-package. Maybe introduce core.py and move the ParameterEstimator and FluxEstimator there?

[adonath]

Thanks @registerrier, so far I have no further comments.