Fix bugs with netcdf handling, add hidden variable
Fix bugs with netcdf handling, add hidden variable

- time variables in netCDF4 iles will be converted
by using own num2date and date2num functions (are
much faster).
- many bugfixes
- add limit_by method
- add rich comparison methods for datetime64 arrays
and strings with datetime information
- make select method more robust and powerful

atmtools/typhon,atmtools/typhon