Slabdip

Fixes an issue where some erroneous values were sampled at the edges of age and spreading rate grids.

**Full Changelog**: https://github.com/brmather/Slab-Dip/compare/v4.2.1...v4.2.2

**Full Changelog**: https://github.com/brmather/Slab-Dip/compare/v4.2...v4.2.1

What's Changed
* Optionally change reference frames by brmather in https://github.com/brmather/Slab-Dip/pull/3


**Full Changelog**: https://github.com/brmather/Slab-Dip/compare/v4.1.3...v4.2

**Full Changelog**: https://github.com/brmather/Slab-Dip/compare/v4.1.2...v4.1.3

**Full Changelog**: https://github.com/brmather/Slab-Dip/compare/v4.1.1...v4.1.2