thanks @jviereck

check signal types in dg.plug, fix #36

While here, develop test from previous commit, and fix another test in
the same file.

fix #36

For now, this test doesn't pass, as an exception is not properly fired,
but we get a segfault instead.

ref https://github.com/stack-of-tasks/dynamic-graph-python/pull/34#discussion_r340106255