MNPE class similar to MNLE

[dgedon]

Implementation of mixed NPE where we have some continuous parameters theta followed by one (or multiple with this PR #1269) discrete parameters. The observation space is fully continuous.

Deprecated mnle.py in net_builders und unified mnle/mnpe as mixed_nets.py.

[dgedon]

Update:

• MNPE and tests are implemented
• for test with Bernoulli prior, I had to change mcmc_transforms to handle discrete distirbution. As default we just compute mean/std for discrete distributions
• currently MNPE with embedding nets does not work yet. Gives some backwards inplace operations error that I couldn't solve yet.