Added support for Waveshare 7.5inch e-Paper V2 (Black and White)

[thecaptncode]

Hi! I very much appreciate your work on the Waveshare driver.
I have added support for the Black and White version of the 7.5 inch e-Paper V2. The specs support both the Black/White and the Black/White/Red V2 models but I did not develop for the Black/White/Red model as I could not test it. Some small changes have been made to your driver to move some methods from the base class to the Epd7In5bc class so I could override them in the new driver. I hope you do not mind. If I can get my hands on other models, I intend to add support for them too. If you find these changes acceptable, please merge them into your repository. If you have any questions or concerns, please let me know and I will address them. Best regards.

[eXoCooLd]

Hi! Thank you for your work! Everything looks good and I merged it into the master branch. But a UnitTest would be nice for the new device :-) Best regards

[thecaptncode]

Thank you very much for merging this revision in. Your architecture was nice and easy to follow. I will work on that UnitTest.

I forgot to mention in the pull request that I added the ClearBlack method to go along with the Clear (White) method because many e-paper screens need to be periodically flashed between solid black and solid white to clean up 'ghosting'. I don't know if that is an issue with either models this project supports but if future models are added, one or more of them may. The Epd7In5 V2 has a feature that automatically shows the new screen as a reverse image before it draws the actual image. That should minimize the need but it still may come up.

By the way, I appreciate that you have this as a nuget package. It makes it quite easy to add e-paper support to a project.

Thanks again.