Fix norm option in plot function in PlotData.

Don't changed stored data when using the norm option in plot.
Fix expand_field function in PlotData.

Don't find additional fields unless the input field type has subfields.
Fix norm option in plot function in PlotData.

Don't divide by data magnitude if it is near zero.
Fix ylabel in plot function in PlotData.
Fix methods depending on kwargs in PlotData/Sample.
Add function to plot Fourier transform in PlotSample.
Merge remote-tracking branch 'hazelnusse/gainschd' into gainschd
Don't print stats for fields with subfields in PlotData.
Fix stats for fields with invalid data.

Use masked arrays to store data, which allows specific a specific datum
to be marked as invalid.
Set the first values of 'computation_time' and
'sample_period' data as invalid.
Change Sample reference to a pointer; minor reformatting.
Merge remote-tracking branch 'oliverlee/gainschd' into gainschd