Tile map

[domfournier]

Allow to invert data misfits on tiled meshes.
Add example

[domfournier]

Here you @jcapriot .
@JKutt This should allow us to start using simulation for the gradiometry inversion.

[codecov]

Codecov Report

Merging #845 into simulation will increase coverage by 0.03%.
The diff coverage is 88.37%.

@@              Coverage Diff               @@
##           simulation     #845      +/-   ##
==============================================
+ Coverage        72.4%   72.43%   +0.03%     
==============================================
  Files             149      149              
  Lines           17693    17735      +42     
==============================================
+ Hits            12810    12847      +37     
- Misses           4883     4888       +5

Impacted Files	Coverage Δ
SimPEG/potential_fields/gravity/simulation.py	81.81% <100%> (ø)	⬆️
SimPEG/maps.py	81.83% <88.09%> (+0.2%)	⬆️

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[jcapriot]

@domfournier Can you add a simple test for the TileMap? It won't work in the bulk test and will likely needs in own test class written.

[jcapriot]

Thanks for getting the example and the test in!

[fourndo]

@jcapriot : Well done on the cleanup. Looking fine here.