findfeatures: finalized the documentation; fixed bug in allocation of matcher...

findfeatures: finalized the documentation; fixed bug in allocation of matcher algorithm if not provided by user; minor improvements to parsing algorithm specification provided by user. Fixes #4556.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7330 41f8697f-d340-4b68-9986-7bafba869bb8