Create a new module to hold volmdlr.core.VolumeModel and avoid circular imports

[GabrielJMS]

• I'm submitting a ...

  • [ ] bug report
  • [x] feature request

• What is the current behavior?

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem Avoid reference to other packages

• What is the expected behavior?

• What is the motivation / use case for changing the behavior?

• Possible fixes

• Please tell us about your environment:

  • branch:
  • commit:
  • python version: