Be less strict about the unit specification in equations

[mstimberg]

Currently, we are very strict in the unit part of an equation leading to unnecessary error messages and making things especially complicated when creating equation strings in some automatic way:

>>> Equations('x : 1/metre')
Traceback (most recent call last):
...
ValueError: "1/metre" does not evaluate to a unit but to a quantity -- make sure to only use units, e.g. "siemens/metre**2" and not "1 * siemens/metre**2"

>>> Equations('x : amp/metre**2')
<Equations object consisting of 1 equations>
>>> Equations('x : amp/(metre**2)')
Traceback (most recent call last):
...
EquationError: Parsing failed: 
x : amp/(metre**2)
        ^
Expected end of text (at char 8), (line:1, col:9)

Should probably be handled in a more general overhaul of the parsing system that also takes care of #308.