Simpeg

Regularization Cleanup
from pr 623. Commits from lheagy, review from rowanc1, fourndo
- ensure that all children of a combo regularization are looking at the exact same regmesh, mappings
- do not store both the mesh and the regmesh on the regularization. Only the regmesh is needed.
- add tests to ensure children of a combo regularization are looking at the same regmesh and mapping objects in memory

Utils Cleanup
from pr 626. Commits from lheagy, rowanc1 , review from rowanc1
- delete util code that has been transferred to discretize so that we are only managing one version!
- changes to `ExtractCoreMesh` were transferred over in simpeg/discretize67. Thanks thast!

Clean up how the regmesh interacts with the combo regularizations
From pr 623. Commits from lheagy, reviews from rowanc1, fourndo
- ensure that all children of a combo regularization are looking at the exact same regmesh, mappings
- do not store both the mesh and the regmesh on the regularization. Only the regmesh is needed. (eg. `reg.mesh` no longer exists, but the mesh used to generate the regularization mesh is still accessible from `reg.regmesh.mesh`)
- add tests to ensure children of a combo regularization are looking at the same regmesh and mapping objects in memory

Remove duplicate utils in favour of discretize
- from pr 626, commits from lheagy, reviews from rowanc1
- delete utils from SimPEG that are in discretize, import the ones from discretize instead
- `avExtrap` has been changed to `av_extrap`

EM Paper
- from PR 614. Commits from: sgkang, lheagy
- bookpurnong inversion examples from the EM paper
- first pass at a grounded source implementation for `TDEM.Problem3D_e`

MT tutorial
- from PR 611, 615, 619. Commits from: thast, sgkang, lheagy. Reviews from: lheagy, rowanc1
- 1D MT layered earth analytic
- updates to the saving directive to output each component of the regularization, option to save the text file or not
- plot the Tikhonov curve with the directive

Testing and Deploy of Documentation
- from PR 618, 620. Commits from lheagy, Review from rowanc1
- cache the docs and examples so that they are only tested on changes rather than testing every time

Minor changes
- update the default for `surface2ind_topo` to be `nearest`. fourndo, review from thast

Highlights
- objective functions and joint inversions (https://github.com/simpeg/simpeg/releases/tag/0.6.0b0)
- Interp Utils, VTEM waveform, Mag Driver updates (https://github.com/simpeg/simpeg/releases/tag/0.6.0.b1)
- examples updates for the em paper
- bug fixes in the surface to ind topo (https://github.com/simpeg/simpeg/releases/tag/0.6.0b2)

See notes in the beta releases for details.

Debugged and improved surface2ind_topo with interpolation options

PR 599 (commits from thast, lheagy, reviews from rowanc1)

- Works on both 3D and 2D tensor mesh.
- User can now choose the interpolation method ('nearest', 'linear', 'cubic')