Simpeg

- commits from: thast, fourndo
- review from: fourndo


This PR addresses issue 761 and follows PR 758 and PR 757 .

There was a naming discrepancy in the `PF` module and that causes me some trouble.
In the `Gravity` module, there are two aliases for `rhoMap`: [`mapping`](https://github.com/simpeg/simpeg/blob/f4d03c44d7bd0369bf73b5b51131bd7f0c6a718d/SimPEG/PF/Gravity.py#L50) and [`mapPair`](https://github.com/simpeg/simpeg/blob/f4d03c44d7bd0369bf73b5b51131bd7f0c6a718d/SimPEG/PF/Gravity.py#L191).
In the `Magnetics` module, there is one alias for `chiMap`: [ `modelMap`](https://github.com/simpeg/simpeg/blob/f4d03c44d7bd0369bf73b5b51131bd7f0c6a718d/SimPEG/PF/Magnetics.py#L255).

This PR settle for `modelMap` as only alias, consistent with the definition of the `LinearProblem` class in `Problem.py`

Additional: Fix mapping name in `LinearProblem` (from `mapping` (depreciated) to `modelMap`)

- from pr 757
- commits from: thast
- review from: fourndo

This formulation takes into account the regularization mapping into the preconditioner and remove the need to check for a specific wiresmap.

- from pr: 758
- commits from: lheagy
- review from: thast

- soft removal of python 2.7: we don't explicitly break compatibility as in 731, but it is no longer tested on Python 2.7
- added a note in the docs saying we will not be working to ensure compatibility with python 2.7 anymore

DC Fields Derivatives
- from pr 724
- commits from: lheagy
- review from: sgkang

DC Resistivity
- add fields derivatives with respect to the electric field (in case you want to invert e-field data)
- add e-field receivers to DC problem
- see for example: https://github.com/simpeg-research/heagy-2018-injection-inversions

TDEM
- add charge and charge density to j-fields
- bug fix in calculation of db/dt in the j-fields object
- add grounded source that solves the mmr problem in the H-J formulation for permeable targets

Directives
- add option to save output to a directory, the default is the current working directory

numpy matlib imports
- from pr 755
- commits from: lheagy
- review from: thast

import matlib explicitly as in numpy 1.16, it isn't imported in the numpy namespace be default


- from pr: 670
- commits from: fourndo, thast
- review from: thast , lheagy

A couple of improvements or fix for Potential Fields:

- Fix reading functions for UBC file format (in case of line break)
- add `ModelMap` property
- Change `survey.nD` definition so it is independent of `Problem`
- Use `reg.deriv2` instead of `reg.W` for computing the preconditioner in `Directives.updatePreconditioner`
- general PEP8 improvements (powered by AutoFormat)
- naming convention: `readUBCgravObs` becomes `readGravityObservations` (for consistency between `Grav` and `Mag` names conventions.)
- Combine all Mag problems (susc, amp, MVI) under Magnetics.MagneticIntegral
- Change coordinate system from legacy x, y, z(down) to convention x, y, z (up)
- Add test+example for MVI-S on TreeMesh
- Add test+example for Amplitude inversion (CSM algo)

- from pr 749
- commits from lheagy
- review from thast

update plot2Ddata
- update logic in plot2Ddata so that two vmin, vmax values cannot be provided to contourf (e.g. it was previously possible that an explicit vmin, vmax was provided by the user and that a vmin, vmax was chosen based on the colorbar)
- ensure that if both clim and vmin, vmax provided to contourOpts that they are identical