Robust setting of initial point

[nabriis]

          I just want to add a tiny comment that is not very relevant to this PR: initial point will be set to a `np.array` type if not specified, through [_get_default_initial_point](https://github.com/CUQI-DTU/CUQIpy/blob/4a07108f2bb3505dfa8b5db2a8b7ba1a1fde766d/cuqi/experimental/mcmc/_sampler.py#L389C1-L391C28); if specified, it will be set to whatever the user gives, a scalar in this case. I wonder if we should add something to check if the user specified initial point is compatible and possibly convert it to a uniform format, like `np.array`. 

A somehow related scenario, with the current test case, if we set the initial point of x to be a list, list(0.5*np.ones(10)), it will lead to an error:

File ~/CUQIpy/cuqi/experimental/mcmc/_gibbs.py:156, in HybridGibbs.sample(self, Ns)
    154 """ Sample from the joint distribution using Gibbs sampling """
    155 for _ in progressbar(range(Ns)):
--> 156     self.step()
    157     self._store_samples()

File ~/CUQIpy/cuqi/experimental/mcmc/_gibbs.py:206, in HybridGibbs.step(self)
    203     sampler.step()
    205 # Extract samples (Ensure even 1-dimensional samples are 1D arrays)
--> 206 self.current_samples[par_name] = sampler.current_point.reshape(-1)

AttributeError: 'list' object has no attribute 'reshape'

Originally posted by @chaozg in #484 (comment)