Make generic weights of adaptative sparse graph

[caballa]

adapt_sgraph.hpp assumes that the graph weights are basic types. The assumption for basic types is because we use memcpy and realloc for efficiency reasons. Thus, to use bignums as weights we should change the implementation which might affect performance. The changes are needed here:

domains/graphs/adapt_sgraph.hpp
domains/graphs/util/Vec.h

This is related to issue #21. Thanks to @elazarg to bring it in!