nco/nco: Shots and power tools

The main new features in 4.6.7 are in the regridder, which introduces a sub-grid regridding option, more automated handling and flexibility with regional grids, and easier flag specification. ncap2 added specialty functions to simplify handling of missing values and in-filling large missing regions with interpolated data. ncatted received an important bugfix for netCDF4 with _FillValue.