Additional Features for CubicHermiteInterp

[alanlujan91]

This PR adds new methods to CubicHermiteInterp.

Because CubicHermiteInterp uses scipy's CubicHermiteSpline, we can easily port additional methods that could be useful in HARK.

These were initially in #1011 but were moved to a standalone PR for further discussion.

These new methods are:

1. der_interp(self[, nu]) Construct a new piecewise polynomial representing the derivative.
2. antider_interp(self[, nu]) Construct a new piecewise polynomial representing the antiderivative.
3. integrate(self, a, b[, extrapolate]) Compute a definite integral over a piecewise polynomial.
4. roots(self[, discontinuity, extrapolate]) Find real roots of the the piecewise polynomial.
5. solve(self[, y, discontinuity, extrapolate]) Find real solutions of the the equation pp(x) == y.

See notebook for more details: https://github.com/alanlujan91/HARK/blob/CHI_new_feats/examples/Interpolation/CubicInterp.ipynb

[codecov-commenter]

Codecov Report

Merging #1020 (2f05061) into master (d59269d) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
- Coverage   72.53%   72.51%   -0.03%     
==========================================
  Files          68       68              
  Lines       10298    10368      +70     
==========================================
+ Hits         7470     7518      +48     
- Misses       2828     2850      +22     

Impacted Files	Coverage Δ
HARK/interpolation.py	42.57% <65.78%> (+1.04%)	⬆️
HARK/ConsumptionSaving/ConsIndShockModel.py	85.86% <100.00%> (+0.01%)	⬆️
HARK/tests/test_interpolation.py	100.00% <100.00%> (ø)

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[sbenthall]

@alanlujan91 can you fix the merge conflict? (sorry for duplicating my comment on #1011 )

@mnwhite would you be able to review this?