`auxdata` directory per user not per environment

[maawoo]

Hi @fabricebrito,
thanks for your work on this conda version of SNAP! I think it's a very valuable development.
I've just started testing it on our university's HPC cluster and it seems to work quite well. One issue I've noticed is the usage of the default .snap directory and the auxdata subdirectory relative to the SNAP installation inside of an environment. It would be much better to set these once for the user, which would avoid the download and accumulation of auxiliary data (DEM, orbit files, etc) each time you start with a new conda environment.

I was able to adjust some configurations to change the default directories, as described here and here. You could also have a look how pyroSAR handles this, as it uses a .snap user directory by default.

What do you think?

Cheers,
Marco