Add coefficient validation method to each model

[jason-ash]

Populate this validation method for each model, checking

• coefficient types
• coefficient shapes (if arrays)
• coefficient value ranges (e.g. positive-only, or relational constraints as in the CIR model.)
• matrix properties (e.g. for correlation matrices)

Then ensure the _validate_coefs is called at the appropriate time before other methods, e.g. sample are able to be called.

pyesg/pyesg/diffusion_process.py

Lines 92 to 94 in ddde7ec

	def _validate_coefs(self) -> None:
	"""Validates shape, type, and ranges of coefficients for the process"""
	raise NotImplementedError()