simpeg/simpeg: NSEM updates

NSEM upgrades Collections of updates focused on improved NSEM data functionality and updating NSEM module to comply with new developments. from pr #515 commits from @grosenkj review from @rowanc1, @lheagy Highlights Compliance with new SimPEG features Uses the SimPEG new objective functions Relying on other latest and greatest SimPEG developments. Addresses some of the long-term development goals in #452 Better plotting Name space clean ups NSEMdata improvements Adds standarad_devation, floor and calc_uncertainies to Data that are indexable in the same way as the data (in parallel with #672 and #202) Adds functions to work and manipulate NSEMData in Utils.dataUtils Plotting methods for Data New interactive NSEMDataViewer for comparing data Improved example 1D forward and inversion 3D foward modelling pep8 cleaning and maintainance and adding documentation. Address issues #631 and #665 Backward compatibility From the current master version of NSEM module, there isn't a large difference and backward compatibility should be intact. Majority of the work in this PR is focused on adding utilities to work and interact with NSEM data, clarifying how to use the code with better examples and do some minor maintenance