Fix/flake8

[flynneva]

• fixed remaining flake8 errors including line length
• fixed remaining pep257 errors

have a look at the line length fixes, if you think they are too difficult to read or make the code less-readible lets discuss. i think all that is needed is an additional file that configures the max-line-length parameter for flake8 (filename .flake8 in in the root dir i think).

additionally, all thats left is to fix the copyright errors. i've moved the legacy copright notice to the LEGACY_COPYRIGHT file in the root directory. there is a ROS2 tool for this called ament_copyright which can "auto-add" whatever license you want. I think we should keep the BSD3 clause license, and for the authors we can curate an AUTHORS file which lists all people who have contributed to the project. what do you think?

[flynneva]

i've gone ahead and replaced all the exceptions with the single custom TransmissionException we have already defined. I think for most of the try/catches this works...but I could be wrong.

I also added a small bit to the README for anyone else who wants to run the flake8 locally.

[whatis777]

LGTM