fix #39

to fix #39 also in Python2

As PyState_AddModule is not yet called in the module definition,
PyState_FindModule will not work, so for Python 3 we need to get the
module from the methods definition to retrieve a pointer to dgpyError.

Fix #39

This adds a command in Custom Entity that acts on the input signal.
Acting on this input signal before setting it should raise an exception
from dynamic-graph.

test_dg_exc was added to check that, and fails for now.

ref https://github.com/stack-of-tasks/dynamic-graph-python/issues/39

While here, fix exception type check in test_bindings.

explicit error in case of misuse of dg.plug, fix #36

As, from the docs:
> Passing arguments of the wrong type (e.g. passing a list when an int
> is expected) should result in a TypeError.

Thanks @jmirabel

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