Move creating folder from `get_folder()` to `init()`

[nils-herrmann]

pybliometrics version: 3.8

Affected classes: All

Expected behavior: init() should create folders not get_folder()

[nils-herrmann]

Here some comments:

• The init() function checks if the config file has all the needed paths, if not the missing paths get written to the config file
• The get_folder() function was moved to the startup.py file
• The CONFIG_DIR variable and get_config_path() function were removed.

[Michael-E-Rose]

PR #317 merged