Generalize code that computes interpolation indexes.
columnSystemCtx improvements.

- columnSystemCtx now owns the fine computational grid and computes it
  using the coarse storage grid when created.
- columnSystemCtx is no longer derived from TridiagonalSystem: it owns a
  pointer to TridiagonalSystem instead.
ColumnInterpolation takes arbitrary grids.
Remove column interpolation code from IceModelVec3.