simpeg/simpeg: Simulation and inversion of time domain IP data

Simulation and inversion of time domain IP from pr: #590 commits from: @sgkang, @fourndo, @micmitch review from: @lheagu Implementation: Use stretched exponential (chargeability, time constant, frequency dependency) for parameterization Tested 2D and 3D SIP problems 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! 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