Fix external dependencies by using VisaInstrument

[reneotten]

Steps to reproduce

1. Import Qcodes
2. Import DecaDAC

Expected behaviour

Regular Import of DecaDac

Actual behaviour

Error since "serial" module not part of dependencies

Suggested Fix

Use the build in VisaInstrument, no need to add further dependencies at this point

System

operating system
WIN64

qcodes branch
feature/FZJ_DecaDAC_driver

qcodes commit
8922738

[lankes-fzj]

The Decadac class now derives from VisaInstrument and uses its methods for the hardware communication. The serial package is not used anymore. --> commit 32bd93a

@reneotten Please try if the driver is working now so that the issue can be closed.