Formatting issues in Python library docstrings

[jarvisschultz]

There are a variety of formatting issues in the docstrings in the Python library. While this isn't a huge deal, it does mean that some of the example usage cannot be directly copy-pasted into a Python interpreter and have it run successfully. This is true even with an indention-aware, "smart" interpreter like IPython's %paste function. Here are a few issues that I noticed:

• Inconsistent indentation on some of the sample code. For example, see ForwardDynamicsTrajectory.
• Mixed usage of tabs and spaces for indentation, even within a single docstring. Python not only discourages use of tabs for indentation, but on Python 3 they've actually disallowed mixed usage. To see this on GitHub, it might help to change the spacing that GitHub renders tabs with. Here's one example with the tab width set to 2 spaces. Notice the inconsistent indentation?
• Some of the more in-depth examples explicitly import modern_robotics but they fail to import numpy or print_function.
• There are a few spelling issues example
• Some trailing whitespace (not a big deal).

[zrwang]

I should look into your pull request before creating one. I found some similar problems mentioned in this issue. I also fixed wrong indentations, especially in the function description comments. Anyway, thanks for your effort.

[jarvisschultz]

@zrwang thanks for your attention to detail!

@HuanWeng can you please review both of these PRs, and accept whichever is more appropriate and then close the other?

[jarvisschultz]

Now that #9 is merged, we can close this issue.