Make TIDY3D_DIR configurable via environment variable

[yaugenst]

Tries to read the TIDY3D_DIR environment variable to set tidy3d's base directory and defaults to the current behavior. Partially solves #1131 and is what I am currently using.

[momchil-flex]

Thanks. Actually we have a related PR #1100 in which the folder is set to /tmp/.tidy3d if ~/.tidy3d cannot be written into. That said we can also add the environment variable option. Note that currently this is only used for the api key, which can also be set through environment variable SIMCLOUD_APIKEY, in which case the config folder should not be needed.

[momchil-flex]

I just merged #1100 ; could you modify your PR to set the default folder as per that modification, when not provided as env variable? Thanks!

[yaugenst]

Oh great thanks! I was aware of the api key variable, but thought it didn't work. Turns out I am bad at reading, I used SIMCLOUD_API_KEY...
That probably makes this PR obsolete for now, it might still be useful though if in the future there are more global configuration options planned.

[yaugenst]

Changed the environment variable to TIDY3D_BASE_DIR to better reflect its purpose according to #1100.

[momchil-flex]

Thanks!