Internal unitless mode
Currently, sympy.Piecewise can't be used with units, it throws some error that numpy.select (which it uses internally) can't deal with values with pint units. We can work around this if there is a kind of 'internal' unitless mode, where maybe the @units.ensure decorator passes all values as magnitudes of SI base units, then performs the calculation, trusts it's going to output the correct unit and returns that. Certain parametrisations like the 桅m Monin-Obukov stability function require piecewise handling (unstable vs. stable conditions) and it would be very helpful and nicely visible in the docs if sympy.Piecewise could be used.