Mask zero points in exp data

[rozyczko]

If a point has a zero value (define zero!), it needs to be masked in the dataset and not used for fitting.
This is to conform to the decision made between the reduction and analysis teams about how to handle zero-intensity points (with undefined varance).

[rozyczko]

masking biases the fit
not clear what the solution is
check other software then - refl1d, genx

[rozyczko]

Currently implemented - masking out for fitting, not for printout.
This is chosen (until told to reimplement) in order to pass the integration tests.