Refactor submodule layout

[jonathf]

Chaospy is getting big, and it the submodule placement could use some care to simplify structure.
From the usage point of view this will affect the lower level functionality by moving some functions upp from root folder and up to submodule. For example chaospy.{quad_fejer -> quadrature.fejer} and chaospy.{orth_ttr -> expansion.stieltjes}. Full overview in the changelog.

[codecov-io]

Codecov Report

Merging #326 (76e5846) into master (634a361) will decrease coverage by 0.2%.
The diff coverage is 97.5%.

❗ Current head 76e5846 differs from pull request most recent head 9050080. Consider uploading reports for the commit 9050080 to get more accurate results

@@           Coverage Diff            @@
##           master    #326     +/-   ##
========================================
- Coverage    98.2%   98.0%   -0.3%     
========================================
  Files         166     173      +7     
  Lines        4962    5066    +104     
========================================
+ Hits         4874    4966     +92     
- Misses         88     100     +12     

[codecov-commenter]

Codecov Report

Merging #326 (256cafe) into master (193413a) will decrease coverage by 0.2%.
The diff coverage is 97.5%.

❗ Current head 256cafe differs from pull request most recent head 340cd8c. Consider uploading reports for the commit 340cd8c to get more accurate results

@@           Coverage Diff            @@
##           master    #326     +/-   ##
========================================
- Coverage    98.2%   98.0%   -0.3%     
========================================
  Files         166     174      +8     
  Lines        4967    5107    +140     
========================================
+ Hits         4879    5006    +127     
- Misses         88     101     +13