Main features:

• Changed ExponentialStatistics name to ExponentialMovingStatistics (Issue Feature Request: Exponentially Weighted Incremental Mean and Variance  #28)
• Implemented time-based discounting for ExponentialMovingStatistics (Issue Feature Request: Exponentially Weighted Incremental Mean and Variance  #28) (incl. Readme)
• Implemented ExponentialMovingCovariance (incl. Readme)
  • https://reasonabledeviations.com/2018/08/15/exponential-covariance/
  • https://vlab.stern.nyu.edu/docs/correlation/EWMA-COV
  • Equivalent of ExponetialMovingStatistics for Covariance and Correlation

Smaller fixes/changes/features:

• Updated Readme for link to Finch Paper for ExponetialMovingStatistics
• Adjusted clear method of ExponentialMovingStatistics: Goes back to state at object construction, does not accept any arguments for modification. I think that fits better to the behavior of the other classes. Modification is still possible over other methods.
• Extended tests for ExponentialMovingStatistics against a true (non-moving) exponential mean and variance formulation

Problems:

I have done my best to resolve all issues that are raised by the CI pipeline. However, I could not resolve all of them. You've been just changing the structure of the project including the CI pipeline and the way Cython is compiled. Thus, I guess you should be able to identify the sources of these problems fairly quickly.

• Integration test fails: Its the lines in the Readme where RunStats_ is used. I don't know why this is happening.

docs/index.rst:4: D000 Unknown target name: "runstats".
docs/index.rst:30: D000 Unknown target name: "runstats".
docs/index.rst:60: D000 Unknown target name: "runstats".
docs/index.rst:85: D000 Unknown target name: "runstats".
docs/index.rst:380: D000 Unknown target name: "runstats".

• Locally running tox also complains in Readme about:

>>> import runstats.core   # Pure-Python
>>> runstats.core.Statistics
 <class 'runstats.core.Statistics'>
Expected:
    <class 'runstats.core.Statistics'>
Got:
    <class 'runstats._core.Statistics'>

When changing in the Readme <class 'runstats.core.Statistics'> to <class 'runstats._core.Statistics'> this does not resolve the issue. Since currently in the main repositories master branch its <class 'runstats.core.Statistics'> I left it that way.

• Test coverage: When locally running tox -e py it tells me that there is 98% test coverage and reports missing lines

402->405, 405, 412->413, 413, 418->419, 419

These are the lines where Exceptions are raised in the new freeze() and unfreeze() methods. However, these are being tested by test_raise_if_not_time_exp_stats()

• I'd recommend merging the PR onto another branch and checking this out yourself.