Scijava spotfeatures plugins

This pull request generalizes the SciJava integration to numerical features.

Feature analyzers for spot, edge and tracks are now discovered thanks to the @Plugin annotation. For feature analyzer, the implementation is much simpler than for detectors.

Spot feature analyzer also introduce the use of a *priority* double value, that is used to force the order in which analyzers are used. This enable having analyzers depending on other analyzers.