Consistently allow user to pass parameter bounds and inits for some but not all parameters

[MoAly98]

A user can currently either provide par_bounds or init_pars to a function for all parameters in the model, or none at all. It would be useful to implement a consistently applied and safe interface for users to provide these values only for a subset of the parameters.

Related discussions:

#512 (comment)
#490 (comment)

Example implementation:

cabinetry/src/cabinetry/fit/__init__.py

Lines 636 to 640 in f2c351f

	par_bounds = [
	user_bound if user_bound is not None else suggested_bound
	for user_bound, suggested_bound in zip(
	par_bounds, model.config.suggested_bounds()
	)

[alexander-held]

somewhat related: scikit-hep/pyhf#2028