Add plot command for stretching method

[janfer95]

With "msnoise plot dvvs" it is now possible to plot the outputs computed by stretch.py in a similar way to the "msnoise plot dvv" command. All arguments should work the same, apart from --dttname and --all, since both do not make sense in that context. I also tried to keep the syntax of the original dvv.py file in the new dvvs.py to make editing and reading the code easier.

I tested it only on the latest pip install msnoise (v. 1.6.1) and I saw that the msnoise 2.0 branch has changed quite a bit, including the plot commands, so there will probably be the need for some adjustments.