Add a function to change the scan order of the sampling steps returned by construct_sampler

[rlouf]

The order in which the sampling steps are chained together can influence the sampling results, we should thus allow users to change the scan order of the samplers we return.

After #76 it will be possible to inspect the sampler graph and identify the sampling steps according to their type. We can then rewire the graphs e.g. as follows:

More generally, we will need to write a series of operators that act on samplers: we may want to change the scan order, randomize the scan order, merge two samplers (e.g. with different scan orders for symmetric scan), vectorize the sampler, draw several samples using scan, etc. Related to #80.