Add support for field measurements

[jonschumacher]

During our discussions on MagneticParticleImaging/MPIMeasurements.jl#27, @jusack and me came across the definition of the field measurement format. Should this be moved over to MPIFiles or stay in MPIMeasurements? Do you see cases where there should be more information stored in the file than what is currently being done? I guess timestamp, currents and fieldsError could be optional.

[tknopp]

Good question. Originally I would have said: No it does not belong into MPIFiles, since MPIFiles ist only for storing MPI measurements. But right now I am not so sure anymore. If we do move it we do need some documentation on the available data formats (which we need anyway). Transfer functions are also stored in hdf5 in MPIFiles.

[jonschumacher]

Alright, I will wait until we have our new gaussmeter and then compare if I see any other fields that might be necessary while acquiring data. Then I will create something and open a PR for a discussion on it.