fix,refactor: build tables correctly. clarify code

changelog:
- `map_samples_to_chemicals.py` now correctly builds tables and will allow `build_script.py` to run to the end (tested with `--samps` flag)
  - redundant functions consolidated (e.g. `combine_chemical_fit_data` and `combine_chemical_dose_data` -> `combine_chemical_data` function with `data_type` arg)
  - variable names changed to be clearer to the end user (e.g. `samp_chem` -> `chem_data`. Most instance of `samp` -> `sample`)
  - added type hinting and docstrings to most functions
  - enforce snake_case for all variable and column names
- enforced formatting on all `src/*.py` modules for building files