Use environment variable for db

[linsalrob]

I added an option to set the database path using the environmental variable SUPERFOCUS_DB.

There are instances when we don't know a priori what the path is and so this allows users to set an environmental variable to define the path.

For example, if super-focus is included in a snakemake pipeline installed via conda, the user can install the databases somewhere and set the environment variable to point to the database location.

To use:

export SUPERFOCUS_DB=/path/to/database/location/

there is now no need to use the --alternate-directory flag.

[metageni]

Thanks @linsalrob