Add inv and derivativeinv to some utility functions

[cc7768]

Adds methods for u^{-1} and du^{-1} for utility functions where it can be computed by hand.

I have some tests that make sure u(u^{-1}) returns what we expect (also have them for du(du^{-1})), but it wouldn't hurt to have someone glance at the equations to make sure I didn't do anything stupid.

[codecov-io]

Codecov Report

Merging #225 into master will increase coverage by 0.04%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   95.85%   95.89%   +0.04%     
==========================================
  Files          25       25              
  Lines        1061     1073      +12     
==========================================
+ Hits         1017     1029      +12     
  Misses         44       44

Impacted Files	Coverage Δ
src/modeltools/utility.jl	95.83% <100%> (+1.38%)	:arrow_up:
src/kalman.jl	100% <0%> (ø)	:arrow_up:

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[sglyon]

General comment -- I think we should consider edge cases in both the implementation and the testing.

For example, what happens at derivativeinv(u::CRRAUtility, x) at x=0?