replace many instances of MutableARGPath by its immutable counterpart

FeasabilityCheck, EdgeInterpolator, ErrorPathClassifier and InterpolationTree now rely mainly on ARGPath, with according changes in the calling classes

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14578 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c