comment out the 3D axes aspect ratio as this is not supported by matplotlib

• 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)

Updates to the linear inversion app

• from pr #20

• commits by: @sgkang

• review by: @dougoldenburg

• updates to the linear inversion notebook

2D crosswell tomography app

• from pr #16

• commits from @sgkang

• review from @fourndo

• add a crosswell inversion app

URL updates

• 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)

New functionality: Gravity notebooks

• 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

updates to potential fields

• from pr #10
• commits from @fourndo, @lheagy

Updates to keep pace with changes in the potential fields API (from simpeg/simpeg#670)