Upgrade to multiple dimensions and binned PDFs

[jonas-eschle]

This moves for asymptotic asimov dataset creation on the zfit binned PDFs and datasets.

Abstracting this away makes it easy to go for multidimensional asimov datasets (as zfit supports it natively) and fixes #55.

Also contains some other fixes.

I've though about the way to do it, whether we wanna replace the unbinned NLL (the self.loss) of the Asymptotic calculator with the Binned one, but I thought it's maybe better to keep it an implementation detail.

What do you think @marinang (let me know if you're not available)

[codecov-commenter]

Codecov Report

Base: 93.51% // Head: 93.05% // Decreases project coverage by -0.45% :warning:

Coverage data is based on head (d50fb77) compared to base (528c2d4). Patch coverage: 91.83% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   93.51%   93.05%   -0.46%     
==========================================
  Files          26       26              
  Lines        1156     1281     +125     
==========================================
+ Hits         1081     1192     +111     
- Misses         75       89      +14     

Flag	Coverage Δ
unittests	93.05% <91.83%> (-0.46%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/hepstats/__init__.py	100.00% <ø> (ø)
src/hepstats/hypotests/__init__.py	100.00% <ø> (ø)
src/hepstats/hypotests/calculators/__init__.py	100.00% <ø> (ø)
src/hepstats/hypotests/core/__init__.py	100.00% <ø> (ø)
src/hepstats/hypotests/exceptions.py	88.88% <ø> (-1.12%)	:arrow_down:
src/hepstats/modeling/__init__.py	100.00% <ø> (ø)
src/hepstats/splot/__init__.py	100.00% <ø> (ø)
src/hepstats/splot/exceptions.py	100.00% <ø> (ø)
src/hepstats/splot/warnings.py	100.00% <ø> (ø)
src/hepstats/utils/fit/sampling.py	78.94% <ø> (-0.54%)	:arrow_down:
... and 22 more

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