Geoscilabs

- This should resolve the failing tests on the `FDEM_Inductive_Sphere`
- I also ran `make format` to run black and fix the formatting (which was also causing a fail on travis)

- from pr 20
- commits by: sgkang
- review by: dougoldenburg

- updates to the linear inversion notebook

- from pr 16
- commits from sgkang
- review from fourndo

- add a crosswell inversion app

- from pr 13
- commits from: lheagy, thast
- review from: thast

Changes
- update image paths to point to this repository instead of gpgLabs, em_apps (closes 11)

- Thanks wwzzyyzzrr for this addition
- reviewers: lheagy and thast

This PR adds two new notebooks:
- `gravityDike.ipynb`: analytic solution for a infinitely long dipping dyke in a half-space
- `gravitySphere.ipynb`: analytic solution for a sphere in a half-space

- from pr 10
- commits from fourndo, lheagy

Updates to keep pace with changes in the potential fields API (from simpeg/simpeg670)