[DOC] Linear Operators

[paquiteau]

This PR improve the documentation of operators/linear/ files. I also took the liberty to clean the import and did some code formatting as well.

[codecov-commenter]

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@@            Coverage Diff             @@
##             develop     #148   +/-   ##
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  Coverage           ?   82.34%           
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  Files              ?       47           
  Lines              ?     2294           
  Branches           ?        0           
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  Hits               ?     1889           
  Misses             ?      405           
  Partials           ?        0           

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unittests	82.34% <0.00%> (?)

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