Need to update backend to use Float32 directly instead of Int16

[natgeo-wong]

ECMWF has updated their datasets on CDS such that the data is no longer provided in Int16 format, but instead directly provided as Float32 data. There is thus a need to change all the backends to natively interact with Float32 formats without needing conversion.

This will require another breaking change from v0.3 to v0.4 (so v0.3 will be the last version that interacts with Int16 formats because I would assume that eventually the Int16 data would be phased out in favour of Float32).

Things to do:

• Go through the code and change all preallocated arrays to Float32 types instead of Int16
• Remove the need for attributes, all missing data is saved as NaN types
• Directly load all into Float32 arrays and not Int16 arrays
• Bump to the next breaking version to denote this change