Generalise the _fix_cell_methods loading callback.

### Something else
The `_fix_cell_methods` loading callback correctly identifies that Iris' cell methods do not always correctly identify the type of processing that has been applied to the cube. However, the current fix of converting 'mean' to 'sum' for specific STASH codes is too generic as the STASH system is capable of various types of processing (off the top of my head, at least mean, minimum and maximum over a defined period).

Ideally this callback would check the UM field codes as defined in UMDPF03 to see what processing was actually applied and assign an appropriate cell method based on that.

Start by locating the `_fix_cell_methods` loading callback and reviewing how it maps STASH codes to cell methods. Consult the UM field codes defined in UMDPF03 to identify the processing applied, including mean, minimum, and maximum over a defined period. Done means the callback assigns the appropriate cell method rather than applying a generic mean-to-sum conversion.

Written by the indexing model from the issue text.