Remove hard-coded paths

[Flaminietta]

We remove any hard-coded paths from within the code by following the suggestion of @paulrougieux: #19 (comment).

All needed input files (e.g., the input matrix) will be stored in a default directory inside the root directory of ProMCDA. However, the user is allowed to change the position of this directory by setting a custom directory path in the environment:

export CUSTOM_DIRECTORY_PATH=/path/to/custom/directory

This allows the user to customize the directory path without modifying the source code of the Python package.

[paulrougieux]

• Great that the directory can be specified with an environment variable.
• Users might have many packages on their systems which require configuration through environment variables. the name of the environment variable should include the name of your package for example PROMCDA_DIR, PROMCDA_DIRECTORY_PATH, or similar.