simpeg/simpeg: Improvements: Potential Fields

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