Interpn

Other
- Squash early development

Changes
* Fix grid cell index selection to properly center the grid cell s.t. t=0 corresponds to index 1
* Add test of cubic method against sine function to capture potential problems not visible when testing against linear and quadratic functions

Other
* Implement cubic interpolation for regular grid
* Continuous first derivative everywhere
* Option to clamp to linear extrapolation to prevent unwanted extrapolation of local curvature
* Test linearized method against linear function in both interpolation and extrapolation
* Test un-linearized method against quadratic function in both interpolation and extrapolation

Changes
* Remove initial guess for cell index from rectilinear method
* Collapse some loops
* Remove support for negative step sizes for regular grid in favor of reducing number of abs() calls
* Remove some saturating sub calls that are not needed now that degenerate grids are not supported
* Get indexing dimension product in the same way for rectilinear method as for regular grid method
* Use better initial value for folds
* Update docs
* Use optimizations for tests because it's faster overall

Changes
* Propagate Result everywhere that unwrap or assert! was being used