Peak and cluster definition parameters

/!\ Code in this pull request is based on #157, please review #157 first.

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 - Create an entity `PeakDefinitionCriteria` to store attributes: `maxNumberOfPeaksPerCluster` and `minDistanceBetweenPeaks`;
 - Create an entity `ClusterConnectivityCriterion` (definition provided by @nicholst in #157) to store the connectivity-criterion used to build the clusters.
 - Both `PeakDefinitionCriteria` and `ClusterConnectivityCriterion` are `used` by `Inference`.

I have a couple of questions:
- How do you feel about those names: `PeakDefinitionCriteria`, `ClusterConnectivityCriterion`?
- Would you prefer to have a single entity storing both cluster- and peak- related parameters? If so, how could we name it?

Thanks for your feedback.

(This pull request follows discussions in NIDASH call on September 22nd ([Minutes](https://docs.google.com/document/d/1VmHvlXFYuFwaZwdou0zhSdtoj1vQjxAGICyeoAKvl8Q/edit?usp=sharing)) and at #157).