Add tests to complete code coverage

[jbeard4]

Add tests to complete code coverage for the following lines, and remove the # pragma: no cover comments.

hobart/hobart/_internal.py

Line 66 in 1162bfe

self.d[v].remove(u) # pragma: no cover

hobart/hobart/_internal.py

Line 82 in 1162bfe

return None # pragma: no cover

hobart/hobart/_core.py

Line 117 in 1162bfe

G.add_edge(e0, e1) # pragma: no cover

hobart/hobart/_core.py

Line 125 in 1162bfe

raise ValueError( # pragma: no cover

hobart/hobart/_core.py

Line 152 in 1162bfe

if ret_pointcloud: # pragma: no cover

Introduced in PR #1