fixed_composition feature

[marcelmbn]

• Include the option of setting a fixed composition (e.g., 3 C, 5 O, 2 H, 1 N; no other atoms)
  • avoids the stupid setting of min_num_atoms and max_num_atoms to appropriate values to implicitly use this "hidden" feature
• Shift check_config into each subclass of BaseConfig so that the Configs can be checked independent of each other but can still be checked together if desired by the ConfigManager
• Moving all GenerateConfig-related checks from the beginning of the actual function to the generalized check_config so that the function that actually DOES things is free of 100 lines of sanity checks
• More tests regarding the composition-based features

[marcelmbn]

Please don't merge immediately after (possible) acceptance of the changes, as I still want to go through it once.

Okay, should be fine from my side.