Get rid of matplotlib dependency from the conversion code

[FrancescAlted]

Part of the conversion code from FLEXDATA format to netCDF4 depends on matplotlib.dates.date2num() function, e.g.:

https://github.com/spectraphilic/reflexible/blob/master/reflexible/conv2netcdf4/helpers.py#L75

Making the conversion code completely independent of a complex plotting library as matplotlib might be desirable (however, matplotlib is heavily used in other parts of reflexible, so not sure it this is worth the effort).