FIX: add a no-p version of ttest_ind

[mmagnuski]

A copy of scipy.stats.ttest_ind, but without p value calculation. Now it assumes with equal_var=False by default (as before)

• add a test (similar to what is in pylabianca)
• try to make the test work with xarrays (maybe axis=axis_idx instead of just (..., axis), which xarray may interpret as dim?)

[codecov]

Codecov Report

Attention: Patch coverage is 80.95238% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.66%. Comparing base (7cff2d9) to head (0d2c465).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
- Coverage   93.77%   93.66%   -0.11%     
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  Files          18       18              
  Lines        2537     2557      +20     
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+ Hits         2379     2395      +16     
- Misses        158      162       +4