remove _fill_holes_xr

[atrisovic]

Similar issue at #12

An example of a file with the flag for missing data (tmax:missing_value = 2.e+20f ;):

dimensions:
        longitude = 1440 ;
        latitude = 720 ;
        z = 1 ;
        time = UNLIMITED ; // (365 currently)
variables:
        float longitude(longitude) ;
                longitude:units = "degrees_east" ;
                longitude:point_spacing = "even" ;
                longitude:modulo = " " ;
        float latitude(latitude) ;
                latitude:units = "degrees_north" ;
                latitude:point_spacing = "even" ;
        float z(z) ;
                z:units = "level" ;
                z:positive = "up" ;
        float time(time) ;
                time:units = "days since 2010-01-01 00:00:00" ;
                time:time_origin = "01-JAN-2010:00:00:00" ;
        float tmax(time, z, latitude, longitude) ;
                tmax:source = "Reanalysis 6-hourly air temperature interpolated to 1.0deg with account for elevation changes, interpolated to 3-hourly and scaled to match the diurnal range and mean of the CRU TS3.0 monthly dataset" ;
                tmax:name = "tmax" ;
                tmax:title = "daily bias corrected maximum temperature" ;
                tmax:date = "01/01/10" ;
                tmax:time = "00:00" ;
                tmax:long_name = "Maximum air temperature" ;
                tmax:units = "K" ;
                tmax:missing_value = 2.e+20f ;
                tmax:_FillValue = 2.e+20f ;
                tmax:valid_min = 193.2268f ;
                tmax:valid_max = 345.4522f ;