MinAtar extension for Pgx

https://www.gnu.org/licenses/gpl-faq.en.html#GPLPlugins

When is a program and its plug-ins considered a single combined program? (#GPLPlugins) It depends on how the main program invokes its plug-ins. If the main program uses fork and exec to invoke plug-ins, and they establish intimate communication by sharing complex data structures, or shipping complex data structures back and forth, that can make them one single combined program. A main program that uses simple fork and exec to invoke plug-ins and does not establish intimate communication between them results in the plug-ins being a separate program.

Note that Pgx just invoke pgx-minatar environments through pgx.make (e.g., env = pgx.make("minatar-asterix")). All MinAtar environment dynamics are implemented in this MinAtar extension repository.