Add Monotone Convex

[alecloudenback]

Todo:

• Make more FM-like (e.g. create associated types and align with fit method)
  • If just given some quotes, needs some model setup to know how many and what timepoints to fit the model to. Should all models come with a setup(model,quotes) function which returns a model set up to be fit? The default definition being setup(model,quotes) = model to avoid needing to define setup stages for models that don't need it.

The aforementioned approach doesn't seem to have negative performance implications in the default case:

• document
• move tests to test suite
• decide which extra methods to support at inception (un-ameliorated, for example)

Based on the paper below

HaganWest Yield Curve Comparison
HaganWest Yield Curve Comparison.pdf

Closes #147

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 129 lines in your changes missing coverage. Please review.

Project coverage is 63.22%. Comparing base (078e19e) to head (d21bdeb).
Report is 22 commits behind head on master.

❗ Current head d21bdeb differs from pull request most recent head 7b3017c

Please upload reports for the commit 7b3017c to get more accurate results.

Files	Patch %	Lines
src/model/Yield/MonotoneConvex.jl	0.00%	127 Missing ⚠️
src/fit.jl	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (078e19e) and HEAD (d21bdeb). Click for more details.

HEAD has 16 uploads less than BASE

| Flag | BASE (078e19e) | HEAD (d21bdeb) | |------|------|------| ||18|2|

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #171       +/-   ##
===========================================
- Coverage   87.43%   63.22%   -24.22%     
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  Files          13       14        +1     
  Lines         366      503      +137     
===========================================
- Hits          320      318        -2     
- Misses         46      185      +139     

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