Add boolean flag to check if walls should be plotted in a room

[matija-marijan]

Hi! Thanks for the wonderful project.

This PR addresses a small annoyance when trying to plot an AnechoicRoom instance, where a [1, 1, 1] dummy room gets plotted, even though it is unnecessary.

I added a boolean flag plot_walls to Room.plot() which either creates or doesn't create the wall PatchCollection.

Corresponding unit tests which check for the existence of such PatchCollection in various scenarios are added under test_room_plot.py.

• Are there docstrings ? Do they follow the numpydoc style ?
• Have you run the tests by doing nosetests or py.test at the root of the repo ?
• Have you checked that the doc builds properly and that any new file has been added to the repo ? How to do that is covered in the documentation.
• Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
• Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".

[fakufaku]

Thanks for the contribution! This should make it nicer to look at.