Added separate classes for b /f hyperedges/graphs

b/f hyperedge classes check that head/tail sets are appropriate
b/f hypergraph classes check that hyperedge set are of correct type
Added class for B-HyperTree and visit function

Implemented a class which checks that a given hypergraph is a hypertree
on construction.
Implemented the function from the "Flows on hypergraphs" paper VISIT for
hypertrees.
initial commit for k-shortest hyperpaths algorithm
version with directedBHyperGraph code
tests for k-shortest hyperpaths algorithm
removed x
Merge branch 'jcadena' of github.com:tmmurali/hypergraph into jcadena
PEP8 compliant
pep8 compliant
Copied code for DirectedBHyperGraph from matt-mohammed
fixed syntax error on conditional
changed xrange to range for python3
Merge remote-tracking branch 'origin/master' into matt-mohammed
Added sample dirbhypertree to data
Fixed identation and syntax for bhypergraph class
Fixed more indentation issues
Changed delimeters for byhypertree to tabs
Added read method for DirBHyperTree
Initial tests for bhypertree
Fixed some bugs with the visit algorithm