Enable user defaults for data storage location

[Sujay-Shankar]

Currently, the user has to manually enter the location of their grid data each time they access a spectrum, unless they happen to conform to "/libraries/raw/PHOENIX" eg., which is the current default. It would be preferable to have a one-time user definition of this location that is then retained and used as the default going forward.

Here we propose a solution, which is the utilization of a .env file to store user-defined metadata such as this. Users would no longer have to repeatedly think about their local directory structure when using gollum.