Active Learning Strategy in BoFire

[jduerholt]

This PR is still work in progress and implements an ActiveLearningStrategy in BoFire as discussed in issue #331.

@jdridder: this is my current status:

• the overall framework is set up, the ActiveLearningStrategy inherits from the BotorchStrategy and can be used in combination with AnyActiveLearningAcquisitionFunction, which is currently only qNegIntPosVar and can be only used with one output feature.
• to make it actually working you have to implement the _get_acqfs method within the functional model of the ActiveLearningStrategy.
• feel free to add more acqfs, if you want to use the ones from @hvarfner PR SCoreBO and Bayesian Active Learning acquisition functions pytorch/botorch#2163, you have to add a validator that they can be used only in combination with a SaasSingleTaskGPSurrogate surrogate.

I would recomment that you just create a branch based on this branch and then create a new PR. Thank you very much for your support.