sizedistribution: get rid of data_fit_normal ... make fit_normal a property that returns the data

[hagne]

• define an updated attribute that says that recalculation is needed

e.g.:
if not _uptodate_fit_normal:
do the calculation
return __data_fit_normal (this is a time series or vertical profile)

def _updated():
_uptodate_fit_normal = False