Use pgsolver from opam instead of bundling it

Simplifies the installation process and allows to compare performance
with other tools that use the same pgsolver version.

.gitmodules

-[submodule "vendor/pgsolver"]
-	path = vendor/pgsolver
-	url = https://github.com/tcsprojects/pgsolver.git

_oasis

@@ -18,7 +18,7 @@ Library libcool
   CompiledObject:   native
   Path:             src/lib
   BuildTools:       ocamlbuild
-  BuildDepends:     libpgsolver
+  BuildDepends:     pgsolver
   CCLib: -lminisat -lstdc++
 
   # Warning: we use C++ sources in .c files because oasis does not recognize
@@ -181,19 +181,6 @@ Document "cool_api"
                       CoAlgReasoner,
                       FunctorParsing
 
-Library libpgsolver
-  CompiledObject:   native
-  Path:             vendor
-  BuildDepends:     TCSLib
-  Modules:          pgsolver/src/paritygame/Paritygame
-                  , pgsolver/src/paritygame/Solvers
-                  , pgsolver/src/paritygame/Specialsolve
-                  , pgsolver/src/paritygame/Transformations
-                  , pgsolver/src/paritygame/Univsolve
-                  , pgsolver/src/solvers/Localmodelchecker
-                  , pgsolver/src/solvers/Stratimprlocal2
-                  , pgsolver/src/solvers/Stratimprlocal
-
 # some cabal similar syntax, so steal its syntax highlighting
 # vim: ft=cabal
 # Local Variables:

pgsolver @ 1c70e798

-Subproject commit 1c70e7981ac2ad5f25b5b378aa1733c62fad242f