Added Sliding Potentiometer & Rotary Angle Sensor

[Propanu]

Added basic support and new classes for the Grove Kit Sliding
Potentiometer and the Rotary Angle Sensor in grove.cxx/grove.h

[arfoll]

Hi & thanks for contributing! Before I can merge this can you change the following things:

• commit needs to be signed-off (notice if the copyright is held by your employer and not yourself privately this should be reflected here at minimum). Something like the following should fix it quickly:

git commit --amend --signoff
git push --force

• message should start with grove: xxx (the commit ammend above will let you do this)
• commit needs to be split into 2 (I can help you with this if you do the above - but a git interactive rebase with 'edit' should do the job)
• no tabs should be present in either of those files
• you need an example to go with your @snippet that needs to be in examples/grovexxx.cxx, see the tutorial for doc on exactly how that works

See the contribution guildelines here

Thanks again!

[Propanu]

Gotcha, I'm making the changes and will submit a new pull once the new/edited commits are in place.
Thanks for all the details!

[rwaldron]

Why? You can add commits to this branch and just push the branch—the PR will automatically update

[Propanu]

Following Brandon's suggestion, I created a new branch with the changes and reverted my master to just mirror upstream. This keeps my local tree neat and clean, making it easier to work on new sensor support.

[rwaldron]

@Propanu Yes. I was mobile and didn't see that your PR was from master.