Add data types 'amplitude' and 'phase'

[prisae]

• [x] Add to receivers the method .data_type
  • complex
  • amplitude
  • phase
• [x] Add to receivers the method .derivative_chain
• [x] Add a data.complex data set:
  • data.complex contains always the complex data
  • data.synthetic contains data in the form of data_type, which is the same type as data.observed
• [x] Add to receivers the method .from_complex; this can be used to obtain the data in the form of data_type from complex data: rec.from_complex(data).
• [x] These changes have to be implemented throughout the Simulation class.
• [ ] Verify implementation and add tests!

[coveralls]

Pull Request Test Coverage Report for Build 1654674648

• 19 of 45 (42.22%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 99.241%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
emg3d/electrodes.py	3	12	25.0%
emg3d/simulations.py	10	27	37.04%
<!--	Total:	19	45

Totals
Change from base Build 1654355318:	-0.6%
Covered Lines:	4184
Relevant Lines:	4216

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💛 - Coveralls

[prisae]

I start to thing that the chain rule for different data types should go to the residual/misfit computation, instead of how we have it now...