Vectors of various types can now refer to each other.

I mean, a vector of a given type can have a function
that has an argument that is a vector of another type.
To achive this, I had to move the template type
definition to another file.
Add function igraph_vector_index_int.

Index a vector with an integer vector, in place.
Add function igraph_vector_ptr_index_int.

Index a pointer vector with an integer vector, in place.
igraph_graphlets now sort results.

According to decreasing Mu coefficients.
Fix an R test case for graphlets().