[DRAFT] Add Rigol instrument

[VitorRamos]

Draft of DS1102E support

[Arkh42]

There is a problem with the code: the DS1102E inherits from Electrometer instead of Oscilloscope. I defined the minimum functions to implement in the oscilloscope.py file. I'm sending a commit ASAP.

[VitorRamos]

Yes, this PR is not complete yet. I created this draft so that we can discuss the implementation details in case anything needs to change.
The DS1102E class, for now, is just a copy of one of the electrometers with the correct query format, the SCPI commands were also not updated yet.
For the oscilloscope class, I ​wasn't sure there's enough difference between an electrometer and an oscilloscope for this project, so I didn't create anything. But in this case, I wait for the base class.

[Arkh42]

The base class is now committed.
You can find it in https://github.com/Arkh42/rtestbench/tree/master/rtestbench/tools/oscilloscope.py

[Arkh42]

I also forgot to mention to write the corresponding test functions. :)