Remove automatic fixing of unit spellings

[saraedum]

Eventually we won't be able to fix all unit spellings to make them match the astropy standard.

We should probably just drop this feature and instead produce a good error message when the unit cannot be understood, i.e., explain to the user in two sentences how units have to be spelled and link to a more detailed documentation on astropy.

[linuxrider]

#127

[linuxrider]

Units can now be written as e.g. "A cm-2", "A cm⁻²", "A / cm2" or "A / cm²" only relying on astropy.