Warn about unrealistic bounds and initial guesses

[vishrutgrover]

Describe the changes you have made in this PR

Adds OsipiBase.osipi_check_bounds_and_initial_guess(), which warns (UserWarning) when user-provided fit settings look unrealistic:

• a lower bound is not below its upper bound
• a bound or initial guess lies outside REALISTIC_BOUND_RANGES — the same ranges already asserted for default bounds in test_default_bounds_and_initial_guesses (D and Dp in mm²/s, normalised signal). This catches e.g. diffusivities given in µm²/ms.
• an initial guess lies outside its own bounds
• the upper bound of D exceeds the lower bound of Dp

The check runs once per object: when an algorithm is initiated via OsipiBase(algorithm=...), it runs in the second __init__ call only, so warnings are not duplicated. It is a warning only, fitting behaviour is unchanged. The generic and body-part defaults raise no warnings.

Example (D and Dp given in µm²/ms):

UserWarning: Possibly unrealistic fit setting: the upper bound of D (5) is outside the realistic range [0, 0.01]. Note that D and Dp are expected in mm2/s and the signal to be normalised.

New tests in tests/IVIMmodels/unit_tests/test_bounds_initial_guess_check.py (16 cases). Full suite locally (macOS, Python 3.11): 711 passed, 0 failed (695 before + 16 new).

Link this PR to an issue [optional]

Fixes #176

Checklist

• Self-review of changed code
• Added automated tests where applicable
• Update Docs & Guides

Contributor: Vishrut Grover