ENH/REF:  Added wage schedule reading.

Forgot it earlier.  Also factored out an elif in a backwards
compatible way.  Added an error check.
API:  Removed ln_dist as an argument for g_p.

Just take it from parameters which needed anyway.
REF: Interp now inherits from PchipInterpolater.

Not sure if this is an insanely bad thing to do...  Need to be careful
with which attrs are available depending on wheter linear vs. pchip.
CLN: reorganize
ENH/BUG:  Better inverse handling if Y is not monotonic.

Theoretically it shoudl be monotonic.  But it was getting
**very** flat at the edges.  This allows for that, resorts, and
gives the inverse.
BUG:  load_params gets grid params from params, not hardcoded.
API:  Fix test for changed parameters to ss_wage_flexible
BUG:  Fixed wage distribution bug.

g_p() was ignoring the wage choices.  This remedies that.

I had to play with the grid; see comment in code.
BUG: Check for NaNs in value function.

This is a tempish fix.  See the branch `handling_grid_errors`
for a more robust/failing attempt.

Here I just widen the grid and check for NaNs.
Ignore the problem and it will just go away.