New example: multimodal.py, to test PTSampler evidence estimation

It's just the example in the docs, plus a little evidence check. Here's the result:

    Estimated log evidence =  -16.9586413385 +/- 7.29507626509
    Analytic log evidence =  -6.67931612501

which means that there is at least one bug in this code! But it runs to completion, and the docs are up to date. What more could one want?