All observers should optionally take targets

[mattngc]

At the moment we have two types of observers, ones that sample isotropically and those that mix isotropic with targeted sampling. At the moment the targeted samples are only user for special cases, but it seems these could be more generally useful.

It might be worth refactoring the observer framework so that all observers can take an optional list of targets for biased sampling. However there should be heavy warnings on the potential implications of biased sampling.