[WIP] First draft for virtual AWG

[shumpohl]

Not final.

TODO:
Expand by a more qutip friendly interface that provides a sampling function.

[coveralls]

Pull Request Test Coverage Report for Build 1692

• 53 of 59 (89.83%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 93.596%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qupulse/utils/tree.py	1	2	50.0%
qupulse/hardware/awgs/virtual.py	51	56	91.07%

Totals
Change from base Build 1675:	-0.05%
Covered Lines:	4843
Relevant Lines:	5106

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💛 - Coveralls

[shumpohl]

I decided to drop the fixed sample rate interface for now as there is no immediate usecase. Instead I implemented a function handle callback. This is the interface needed for qutip integration.