add unload_modules to compas_blender

[beverlylytle]

As a developer working in Blender I want a utility function similar to compas_rhino.unload_modules so that I can quickly iterate changes in the source code (without quitting and restarting Blender). This pull request is to address that. It is the same as the compas_rhino unload_modules except that the filter object is cast to a list because Blender complains that the dictionary sys.modules changes size while being iterated over. While there is a docstring I didn't add it to the documentation, mimicking what exists for compas_rhino.

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[tomvanmele]

LGTM. just the py27 stuff...

[tomvanmele]

can be removed. only necessary for Rhino because of Python 2.7...

[beverlylytle]

oh, of course. old habits die hard