ENH: Allow for custom grids

[timmens]

Feature: Allow for custom grids besides linear and logarithmic

Potential use cases: Kinks in the tax schedule, etc.

Implementation

Passing the custom grids to LCM's internals will be easy. The complexity of this feature stems from the fact that we need to write a get_custom_grid_coordinate function, that returns a generalized coordinate value for the interpolation.

[timmens]

Incorporated in #16