Recipemd

- *Fix:* Scaling of amounts with `None` factors
- *Fix:* Possible infinite recursion when flattening recipes
- *Fix:* Make `recipemd` compatible with Python 3.9

- *Fix:* Duplicating ingredients in groups as top level ingredients when multiplying recipe
- *Fix:* Inconsistent changelog formatting

- *Fix:* Flattening failing with exception

- *Fix:* Incorrect trove classifiers

- *Breaking*: Create separate lists of ingredients and ingredients groups in `Recipe` and `IngredientGroup`. This
ensures that ingredients groups always follow after ingredients (see https://github.com/RecipeMD/RecipeMD/issues/6)
- Improve error message when trying to flatten a linked recipe and no matching yield can be found.
- License under LGPL

- Add `-v/--version` argument to `recipemd` and `recipemd-find` to display current version