Add ascendingand descending versions of the 7 parameter richards curve

[caroott]

This pr adds two versions of the Generalized logistic function with 7 parameters. This includes the parameters Q, which is related to Y(0) and M, which can be thought of as starting time for the growth, when compared to the 5 parameter version.

[bvenn]

In module GrowthModels there now exist a richards and richardsAscending model. Since richards is ascending as well, I would recommend to change those to either richards4Param and richards7Param or to richards and richardsGeneric

Could you additionally please provide three simple parameter examples of your model at: https://fslab.org/FSharp.Stats/GrowthCurve.html#Model-examples

[codecov-commenter]

Codecov Report

Merging #146 (8bf7642) into developer (bf406ee) will increase coverage by 0.39%. The diff coverage is 100.00%.

:exclamation: Current head 8bf7642 differs from pull request most recent head 599eaa3. Consider uploading reports for the commit 599eaa3 to get more accurate results

@@              Coverage Diff              @@
##           developer     #146      +/-   ##
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+ Coverage      28.32%   28.71%   +0.39%     
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  Files            112      112              
  Lines          11966    12032      +66     
  Branches        2546     2546              
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+ Hits            3389     3455      +66     
  Misses          8469     8469              
  Partials         108      108              

Impacted Files	Coverage Δ
src/FSharp.Stats/Fitting/NonLinearRegression.fs	67.58% <100.00%> (+3.42%)	:arrow_up:

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