remove support for $SNOWGLOBES variable to use custom SNOwGLoBES directory

[JostMigenda]

as we're using snowglobes_data, we can drop the support of $SNOWGLOBES variable. If a user needs to use a custom snowglobes, they can use pip install $SNOWGLOBES/snowglobes_data to use their package in snewpy.

Originally posted by @Sheshuk in #370 (comment)

[JostMigenda]

Before we had snowglobes_data available as a separate package, setting the SNOwGLoBES directory via an environment variable was useful; now that we do have a separate package, keeping the environment variable around to override the installed package can be confusing. As @Sheshuk highlighted in #370, it can even lead to bugs if people update to a newer version of snewpy but forget to update the version of SNOwGLoBES.

Since I don’t see a significant advantage of using the environment variable to point to a custom version of SNOwGLoBES over pip-installing that same custom version, I’d be in favour of removing support for the environment variable.