Save noise estimate as custom parameter to MDF on calib processing

When calculating the snr for the system matrix (in `calculateSystemMatrixSNR`) we calculate a noise estimate anyways. We should save it for future use e.g. in the whitening weighting during reconstruction.

We could add a getCalibNoise(f::MPIFile, frequencies) function that either reads that parameter or calculates it using getMeasurementsFD

Probably useful to be combined with #126 