Initial commit

contains only graph theory stuff written by Lars Noschinski
Add initial theory file
removed is_box_def
add additional files from Graph_Theory afp

have definition for subgraphs
prove union of Sgraphs is Sgraph
added basic stuff for matching and subtraction
stuck on proof of subtract - not provable?
proved string_graph_complement (subtraction)

discuss no_dangle assumptions
fixed wire_box condition and added empty graph
encoded and added some basic graphs

did experiments to see how fast checking can be done. It's very
very slow currently. Problem lies in proving string_graph locale
in particular the box-head conditions are very slow to be proven
(I suspect). Proving pseudo_digraph is relatively fast
(2-3 seconds)
see previous commit - forgot to add files
fixed performance issues

changed some of the assumptions in the locales
replaced them with FOL sytax, instead of meta-logical syntax
it appears now that the slowest part is subgraph checking
cleaned up code a little bit - trivial changes
added R1 graph to example

computing the pushout is straightforward in isabelle
did some stuff for isomorphism definitions

performance problems for proving equality of tail / head
Proved that iso is an isomorphism
typo
moved code around, defs for H1 G1, labels for lemmas
add app_iso lemmas
simple reorganization of lemmas into subsections