parse_units instead of parse_expression

[TomNicholas]

Well done on releasing this guys!

I just gave it a trial by fire on my actual data, and found a bug. My data already has units stored in the attributes, written like 'm^2' and so on.

But the quantify method fails with ValueError: Integers to negative integer powers are not allowed when I try to convert something like m^-1. Turns out this is pint, not pint-xarray:

In [30]: ureg.parse_expression('m')                                                                                                                                                           
Out[30]: array(1) <Unit('meter')>

In [31]: ureg.parse_expression('m')                                                                                                                                                           
Out[31]: array(1) <Unit('meter')>

In [32]: ureg.parse_expression('m^1')                                                                                                                                                         
Out[32]: array(1) <Unit('meter')>

In [33]: ureg.parse_expression('m^0.5')                                                                                                                                                       
Out[33]: array(1.) <Unit('meter ** 0.5')>

In [34]: ureg.parse_expression('m^-0.5')                                                                                                                                                      
Out[34]: array(1.) <Unit('1 / meter ** 0.5')>

In [35]: ureg.parse_expression('m^-1')
ValueError: Integers to negative integer powers are not allowed.

@jthielen maybe you know a good reason why it does this? I can't see one...

But it seems it can be solved just by using parse_units instead of parse_expression in _decide_units. I think that's the more appropriate choice of method to call anyway - if we're parsing the str attribute of a DataArray we expect to be parsing a unit, not a mathematical expression.