fixed filters for GetListOfMatrixAsString

Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `GetListListOfStringsOfMatrix' on 2 arguments called from
GetListListOfStringsOfMatrix( Eval( M )!.matrix, R ) called from

reason: IsMatrix( [ [ 1 ], [ eps^3+eps^2 ], [ 1 ] ] ) returns false

reported by:

From: Sebastian Posur <sebastian.posur@rwth-aachen.de>
Subject: Bug in ZerothTateDifferential
Date: 3. Juli 2013 21:16:34 MESZ
To: Mohamed Barakat <mohamed.barakat@rwth-aachen.de>

Hallo Mohamed,

bei Experimenten bin ich auf den folgenden Fehler in ZerothTateDifferential gestoßen,
hast du eine Idee, was da passiert?

...

Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `GetListListOfStringsOfMatrix' on 2 arguments called from
GetListListOfStringsOfMatrix( Eval( M )!.matrix, R ) called from
GetListListOfStringsOfHomalgMatrix( M, R ) called from
GetListListOfHomalgMatrixAsString( M, HomalgRing( M ) ) called from
GetListListOfHomalgMatrixAsString( M ) called from
SaveHomalgMatrixToFile( filename, M, HomalgRing( M ) ) called from
...  at line 20 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>