Add fill with NaN option in bindGrid

[jbedia]

Add a fill.with.NA logical flag in bindGrid(..., dimension = "time") in order to fill with NA values when the resulting binded time series is not regular. Needed for conversion to ts in other climate4R wrappers (e.g., drought4R, etc.). I suggest an internal helper fillNA, performing the more or less opposite operation as the already existing filterNA to this aim.