Implement Generic Pebbler, climbOnce method for AbstractPebbler and new Pebble Orders: Distance, WasPebbled, InSubProof, abstract DecayOrder and ChildrenDecayOrder

Generic Pebbler gets a list of strings, which represent an ordering which decides ties according to the entered orderings

climbOnce methods is the first step torwards using search heuristics for pebblers

Distance order calculates spheres around nodes and checks if a node in the sphere is pebbled

InSubProof order counts in how many subproofs one node occurs

DecayOrders not only use a calculated measure from the node of question, but also from its premises. There are parameters for decay factor, maximum depth and function how the measures from the premise are combined

ChildrenDecayOrder is a decay order that measure the (negative) amount of children of a node