add optional data_path parameter to generate_spectra_splits

[aaronwtr]

added an optional data_path parameter that can be used to specify path where the spectra splits can be saved to. If left empty, the folder gets put in the root directory, as it was before. Beware: when specifying location, make sure to format the path as <path/to/folder/>, with a final /. This is necessary to preserve the original implementation where the folders get written to the root folder without adding to much additional logic.