Have volumes as a separate object

[smuu]

Currently, volumes are added to instances with instance.AddVolume(size).

I propose to add a Volume object, that can be added to an Instance with instance.AddVolume(volume).
This enables adding the same volume to multiple containers in one sidecar.

Tasks

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1. Adding volume should also be possible with mounting to multiple containers in Pod #167
   enhancement knuu +2
2. Add the feature to add multiples volumes #317
   enhancement good first issue help wanted knuu +4
   

[smuu]

Related: #167

[smuu]

Related: #317