Implement join() method to combine multiple uncertainty quantifications

[luisfabib]

Implements a new method join() in the UQResult objects to combine multiple UQResult objects into a new extended object. This allows to combine different uncertainties of parameters fitted separately but contributing to a common model, allowing a proper uncertainty propagation.

The function takes an arbitrary number of UQResult objects and returns a new one:

combined_uq = param0_uq.join(param1_uq, param2_uq, param3_uq, ...)