Simpeg

- originally from pr 698
- commits from: prisae
- review from: lheagy, rowanc1

`SimPEG.versions()`
- Print the versions of packages required by SimPEG
- adapted from that implemented in [empymod](https://github.com/empymod/empymod)
- Fixes 697



- from pr 695
- commits from: lheagy
- review from: sgkang

Summary
- move the if else so that we don't have to repeat the rx if / else statement in getABMN

- from pr 693
- commits from sgkang
- review from lheagy

Summary
- small bug fix in indentation level of the return statement in `get_dpred` in `invProblem.py`

Bug fix in the L2Objective function deriv2

- From pr 692
- commits from: lheagy
- review from: rowanc1

Summary

fix deriv2 on the L2Objective function if v is None, add a test to ensure behaviour is the same if v is none and if v provided

- closes issue 686

Simulation and inversion of time domain IP

- from pr: 590
- commits from: sgkang, fourndo, micmitch
- review from: lheagu

Implementation:

1. Use stretched exponential (chargeability, time constant, frequency dependency) for parameterization

2. Tested 2D and 3D SIP problems

3. Deprecate `Multiregularization` and use combo objective function, so pulled `ref/objectivefunctions` branch - This is for inverting multiple parameters. Thanks lheagy and fourndo for combo objective functions, and rowanc1 for wire implementation!

4. Implement `storeJ` option for both IP and SIP problems (2D and 3D), which boost up speed for relatively small problems.

Example:

True model

Recovered model

Data fit at the fist time channel:

- from pr 690 (supersedes 689)
- commits from lheagy
- review from dccowan and fourndo

Summary of changes
- ensure `indActive` set prior to cell weights in the regularization init so the validator looks for cell weights that are the same size as the indactive.
python
previously, most usage was the following
reg = Regularization.Simple(mesh=mesh, indActive=indActive)
reg.cell_weights = cell_weights

this should be equivalent but prior to the pr, this use-case failed
reg = Regularization.Simple(mesh=mesh, indActive=indActive, cell_weights=cell_weights)

- fixes the bug dccowan was encountering in the example he generated in 684