Incorrect search space generation using very large search space (>1,000,000)

[sevenroediger]

The create_reaction_scope function in scope_generator.py does not work properly when the search space size exceeds the chunk_size (default setting of 1,000,000).

In that case, the DataFrame, which contains the search space and is returned by the function, will only contain parts of the actual search space.
When the chunk_size is exceeded, the function will go through the reaction setting combinations in chunks, but instead of adding the chunks to the previously generated DataFrame from prior chunks, it will overwrite the DataFrame so that it only contains the last part of the data.

This part of the function only operates if the search space is larger than 1,000,000 combinations. Otherwise, all data will be written at once and the search space is generated correctly.