Add /models/statistics

[haakonvt]

Adds:

• /models/statistics
• /models/statistics/spaces
• /models/statistics/spaces/byids

TODO:

• docstrings
• tests
• add mocks
• wait until this PR is merged: https://github.com/cognitedata/service-contracts/pull/2499

[erlendvollset]

Nice! Let's hold off on this until we publish the API though. I don't think we need to break anything, but I'm not 100% sure. Let's do a sync next week.

[haakonvt]

How pandas viz in notebooks:

(could potentially be nicer? a lot is hidden in 'instances)

The other I'm pretty much happy with:

[codecov]

Codecov Report

Attention: Patch coverage is 75.63025% with 29 lines in your changes missing coverage. Please review.

Project coverage is 90.61%. Comparing base (73fe764) to head (2bed274).

Files with missing lines	Patch %	Lines
...te/client/data_classes/data_modeling/statistics.py	73.49%	22 Missing ⚠️
cognite/client/_api/data_modeling/statistics.py	74.07%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2095      +/-   ##
==========================================
- Coverage   90.69%   90.61%   -0.08%     
==========================================
  Files         149      151       +2     
  Lines       22880    22996     +116     
==========================================
+ Hits        20750    20838      +88     
- Misses       2130     2158      +28     

Files with missing lines	Coverage Δ
cognite/client/_api/data_modeling/__init__.py	100.00% <100.00%> (ø)
cognite/client/data_classes/data_modeling/ids.py	92.61% <100.00%> (ø)
cognite/client/testing.py	100.00% <100.00%> (ø)
cognite/client/utils/_pandas_helpers.py	86.04% <100.00%> (+0.33%)	⬆️
cognite/client/_api/data_modeling/statistics.py	74.07% <74.07%> (ø)
...te/client/data_classes/data_modeling/statistics.py	73.49% <73.49%> (ø)

... and 2 files with indirect coverage changes