Merging the phenology data type file with casa_variable.F90

At the moment, all the CASA derived data types except the phenology are contained in `src/science/casa-cnp/casa_variable.F90`, except the phenology, which is in `src/science/casa-cnp/casa_phenology.F90`. The phenology file doesn't contain any additional functionality, so it would be a good idea to move it to the `casa_variable.F90` file so they're all in the same place.