Multilevel doc branch mergin into v0.50.0-release

Sending the documentation on multilevel method.
The user's manual now reflects the new classes names (without 'uq' and 'Class'), there are new class diagrams and some collaboration graphs. There are descriptions of the Metropolis-Hastings, Multilevel and Monte Carlo methods, together with their algorithms.
The classes MLSampling, MLSamplingOptions and MLSamplingLevelOptions are now documented.