Add ProcessSteps for masking

[ahoermann]

This PR proposes to add two ProcessSteps related to masking.

1. ApplyMask: The signal and optionally other BaseData are modified in-place, setting the values where mask = 1 to zero.
2. ThresholdMask: A new mask is created based on whether the values in signal are greater than threshold.

Both steps have been tested qualitatively but not formally with pytest.

[ahoermann]

Added tests for the new ProcessSteps, 100% coverage

[toqduj]

Threshold masking looks good, should probably have both a lower and upper threshold (dectris marks invalid pixels with a negative integer), and should be applicable to not just signal but any specified BaseData reference. The latter is useful if we want to, for example, mask pixels where the flatfield correction matrix exceeds a certain band. All improvements though, so feel free to leave for later (or leave for me).