Consistently use the test sets as reference for gap_before and gap_after

[WenjieZ]

There are two ways of defining a derived cross-validator. One is to redefine _iter_test_indices or _iter_test_masks (test viewpoint), and the other is to redefine _iter_train_masks or _iter_train_indices (train viewpoint).

Currently, these two methods assign different semantic meanings to the parameters gap_before and gap_after. The test viewpoint uses the test sets as the reference:

train    gap_before    test    gap_after    train

The train viewpoint uses the training sets as the reference:

test    gap_before    train    gap_after    test

This diverged behavior is not intended inappropriate. The package should insist on the test viewpoint, and hence this PR. It will be enforced in v0.2.

I don't think this issue has touched any users, for the derived classes in this package use _iter_test_indices exclusively (test viewpoint). No users have reported this issue either. If you suspect that you have been affected by it, please reply to this PR.

[codecov]

Codecov Report

Merging #45 (fb26eda) into master (2abbc3d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   97.51%   97.51%           
=======================================
  Files           3        3           
  Lines         643      643           
=======================================
  Hits          627      627           
  Misses         16       16           

Impacted Files	Coverage Δ
tscv/_split.py	93.77% <100.00%> (ø)
tscv/tests/test_split.py	99.74% <100.00%> (ø)

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