Convenience functions (+update of the check package to github)

Few Convenience functions for Matrix manipulation + their tests.
I have not changed the API (yet?).

I also updated the check package, that has migrated to github.
Sligthly true :-) [Bis is the old implementation:]
BenchmarkIsSym10Sym	 5000000	       358 ns/op
BenchmarkIsSymBis10Sym	 5000000	       469 ns/op
BenchmarkIsSym100Sym	  100000	     29159 ns/op
BenchmarkIsSymBis100Sym	   50000	     34836 ns/op
BenchmarkIsSym1000Sym	     500	   6987846 ns/op
BenchmarkIsSymBis1000Sym	     500	   8343484 ns/op
BenchmarkIsSym3000Sym	      20	 105254446 ns/op
BenchmarkIsSymBis3000Sym	      20	 101960107 ns/op