Improve added noise

[prisae]

Will close #231

ToDo's

• [x] Change to uniform random phase, constant amplitude (or give the option for either; but if Gaussian to Re/Im, only one random realization [not independent]).
• [x] Add option for non-zero mean.
• [x] Make random_noise an own attribute in emg3d.surveys.Survey, with the possibilities to
  • ~return noise~
  • add to existing dataset
  • save as dataset
• [x] Make noise and chop_noisefloor optional with Simulation.compute(observed=True).

Implemented; tests missing.

=> https://curvenote.com/@prisae/emg3d-as-solver-for-simpeg/randomnoise-f-domain-csem

[coveralls]

Pull Request Test Coverage Report for Build 1070342127

• 30 of 30 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0005%) to 99.853%

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Totals
Change from base Build 1057436115:	0.0005%
Covered Lines:	4067
Relevant Lines:	4073

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💛 - Coveralls

[prisae]

Missing https://empymod.emsig.xyz/en/latest/gallery/educational/random_noise_f_domain.html