Modified OstirConstants and a Python version of calibration script (#13)

* Modified and added new features to the dataclass OstirConstants:
- dG_spacing_constant_push/pull can now be defined when initiating a OstirConstant object
- user can call `constant.refresh()` to re-calculate RT_eff, K, and enforce correct data type. It is automatically called after creating a OstirConstant object via `__post_init__()`.
- improve I/O handling, including using a more user-interpretable __str__ for printing (__repr__ is not changed), and read from / write to json file.

* created a python version of the R script ostir_calibration.

* Remove extra calibration files

Housekeeping to remove two files generated by the python
calibration notebook which are functionally equivilant
to the R generated files

---------

Co-authored-by: croots <[email protected]>