Resolve "offset_add is not behaving as expected"

Closes #210

See merge request !1

replaced index constructors from specific signed / unsigned integers by template versions; should be more general and fix a compilation error on macOS (relates #205)

TTNetwork::use_dense_representation and exporting dense_copy and sparse_copy to python; signal cascade example in documentation

a word about thrad safety; linking to first entry in doxygen if there are multiple; some more link fixes