Parsing a in^2 unit results in a `SyntaxError: Value expected`

**Describe the bug**
When parsing a unit that is negative and includes a power of `in`, a `SyntaxError: Value expected` is returned.

**To Reproduce**
1. Go on the main website and type `-2in^2` or `-1in^3` or `-2 in^4` in the demo box.
2. Click "Evaluate"
3. See a `SyntaxError: Value expected` pop up.
