Implement caching

[huaracheguarache]

A very modest stress test revealed just having 3 users access the tool at the same time causes unacceptably long loading times and makes the tool unresponsive. Some form of caching has to be implemented to address this issue.

[TomLav]

This should be first handled by adding pre-processed fields (e.g. ranks, climatology, etc...) to the netCDF product file of the OSISAF. The visualization tool would then only have to plot the fields that are in the files.

[huaracheguarache]

Notes from last meeting: @TomLav wants to be able to take a look at the netCDF files himself to decide whether the structure is acceptable or whether any changes need to be made.