exponent handling

[bc0n]

ixTape = wTape*tTape^3/12, where wTape = mm needs to just work. But this gets confused, as ^ would need to be defined for all int64 but return different types according to its value....Base.:^(x::MilliMeter, y::Int64) = if y == 2 x^2, y==3 x^3, ...

[bc0n]

And there is no way to search for powers of an existing unit, or relations containing it. It seems like allUnitTypes needs to be replaced with a graph, with edges as operations.
But this will still be tricky type unstable, as the return type depends on one of the input arguments and I don't think you can dispatch on arg value, y::Int64==2 or something.

Maybe there's a way to expand integer ^ into aaa...? But the better solution is likely found along #26.

[bc0n]

Limited but can explicitly define for now:

function Base.:^(x::T, y::Int) where T<:AbstractMeasure 
  if y==2
    return x*T(1.0)
  end
  if y==3
    return x*T(1.0)*T(1.0)
  end
end