Simpeg

Use of sparse inversion for non-linear inverse problems (DC, TDEM, FDEM),
where sensitivity matrix is changing in the inversion iterations.

This can be quite expensive when sparse inversion is used, because
we need a number of iterations to make sparse inversion converged;
requires a number of call for computing sensitivity matrix.

We are doing a simple cheat here. Sparse inversion starts when Smooth inversion finishes.
So, we assume that our inversion is close to the solution, and hence sensitivity matrix does not
change that much after while updating for sparse inversion.

By passing `fix_Jmatrix=True` to `Update_IRLS`, you can fix sensitivity matrix, that is we are using the same sensitivity matrix for all sparse inversion iterations.

- review from: fourndo and lheagy
- commits from: sgkang

- from pr 691
- commits from lheagy
- review from sgkang

Updates
- work on simplifying the storage of the inner product matrix derivatives
- implement storage of the mu matrix derivatives

- 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