Bug: can perform `numpy.linalg.solve` on offset unit

Offset units (such as `degC`) shouldn't be allowed to perform `numpy.linalg.solve`. However, in current pint, the following code doesn't throw error:
```python
from pint import UnitRegistry;ureg=UnitRegistry();import numpy
numpy.linalg.solve(ureg.Quantity([[1,2],[3,4]], "degC"), [1,1])
```
and the result is `<Quantity([-1.  1.], '1 / degree_Celsius')>`