Arbitrary source shape

[prisae]

Improve emg3d to work with arbitrarily shaped sources, which includes loops (hence "magnetic" sources") and also magnetic receivers for the adjoint-state gradient (which requires magnetic sources).

=> #101, #132

TODOs

• Arbitrary source shape.
• loop-shortcut for arbitrarily rotated unit loops.
• Implement mrec/msrc in adjoint-state gradient => comes in another PR later.
• Check source strength (strength=[0, 1, x]) !
• Fix bug in get_source_field depending on grid

[coveralls]

Pull Request Test Coverage Report for Build 472294711

• 47 of 47 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0003%) to 99.953%

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Totals
Change from base Build 412651663:	0.0003%
Covered Lines:	4264
Relevant Lines:	4266

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💛 - Coveralls