rename Data.linear_fits -> .cycles_lin_fits

- update save_cycles_fits(), _fit_stress_strain_cycles()
- update docstrings of fit_stress_strain_cycles(),
  fit_stress_strain_cycles_list()
- update mark_cycles_fits()
- update & rename plot_fits_stiffness() -> plot_cycles_fits_stiffness()
update fit_stress_strain() for arbitrary number of strain regions

- update Data:
  - remove .irange_small, .irange_large, .linear_fit_small, .linear_fit_large
  - new .strain_regions_iranges, .strain_regions_lin_fits
- update & rename save_fits() -> save_strain_regions_fits()
- update detect_linear_regions(), set_strain_regions()
- update & rename mark_fits() -> mark_strain_regions_fits()