changing parallel_cores default to 1

[pdevis]

changing the default to parallel_cores=1, and adapting the docs.

Also includes commit that allows objects to be returned as MC sample (We needed this for CHIME uncertainty propagation). This can be useful for measurement functions that are not flexible, and that return any object, rather than a numpy array. This option can only be used when separating MC propagation in different stages, for the run_samples step. See https://punpy.readthedocs.io/en/latest/content/punpy_memory_and_speed.html#separating-mc-propagation-in-different-stages