Custom number of pumps

[aisbergg]

Instead of hard coding the number of pumps to be used it should be dynamically configurable in the config file. So then only those pumps that are actually defined in the config are initialized. Simple reason is that I want to deploy the cocktail mixer at the DiK which contains only 6 pumps. I want to simply disable the unused slots, so the refering GPIOs can be used otherwise.

[bockha]

see also #34
@goergch: extend the MixerControl to support an arbitrary amount of pumps