Move the isoutlier function/methods to a separate file.

[joa-quim]

Move more files to "extras"

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.24242% with 17 lines in your changes missing coverage. Please review.

Project coverage is 85.91%. Comparing base (c9d8e0c) to head (6af53a5).

Files with missing lines	Patch %	Lines
src/extras/isoutlier.jl	72.91%	13 Missing ⚠️
src/trend1d.jl	82.35%	3 Missing ⚠️
src/common_options.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1710      +/-   ##
==========================================
+ Coverage   85.90%   85.91%   +0.01%     
==========================================
  Files         166      167       +1     
  Lines       23345    23362      +17     
==========================================
+ Hits        20054    20072      +18     
+ Misses       3291     3290       -1     

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