Add prior args for MAP cost function

[BradyPlanden]

Feature description

Update the MAP cost function to allow for prior to be passed on construction. Keep the current functionality that defines these from the Parameters class, but only if the default priors arg is passed.

Motivation

No response

Possible implementation

Functionality should look like this:

prior1 = pybop.Gaussian(0.1, 0.01)
prior2 = pybop.Uniform(0.5, 0.6)
cost = pybop.MAP(problem, prior=[prior1, prior2])

Additional context

No response