delete the check isinstance(calc, BaseCalculator) in the Mixer class

delete the check isinstance(calc, BaseCalculator) for more pythonic duck typing, also more consistent with e.g. setting atoms.calc = calc. The init method still requires an implemented_properties attribute, so e.g. the asap EMT still does not work yet

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• "closes #XXXX" is in the body of the MR description to link the related issue, if applicable.