ENH: Validate and improve interpolation

[timmens]

There are a few issues regarding interpolation on grids that need to be validated and/or improved:

• Validate that the behavior of the interpolators when called on values outside the grid is well-documented, correct, and behaves according to the "law of least surprise".
  Note: Follow discussion in Update grids.py and interpolation.py #69.
• Check whether RegularGridInterpolator could be an improvement over map_coordinates for the interpolation.
• Add possibility 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.