i-191: output netCDF format broken for MFDataset to single netCDF

MFDataset could not properly be written to netCDF due to the format
coming back as a list of strings. The condition is now caught and the
first format is chosen.
i193: remove sample size as default output for calculations

A new parameter called calc_sample_size was added to OcgOperations.
The default (False) means sample sizes will not be computed for
calculations.
i-196: support ECA index calculations for large arrays

Implemented new function class to computed dynamic percentile
thresholds with a moving window.
Merge branch 'i196-eca'