modified Valon_5015 and add Vaunix LDA ethernet access

[kto-ts4]

Modified Valon_5015.py

• Previously Valon_5015.py does not work because of failure in _flush() method.
• Which buffer to be cleared was changed. Not sure this change is correct but works well now.

Add LDA_eth.py

• It is used for connection with Vaunix digital attenuator via ethernet.
• Based on LDA.py, rewriting some methods to use DLL by the vendor.

[jenshnielsen]

@kto-ts4 Was there a specific reason for closing this. We are a bit behind on review but we will get to all prs eventually

[kto-ts4]

@jenshnielsen I noticed that I missed to include Jupyter notebook example that is required in the readme. If it is OK, I will reopen this PR.