Exponential should work with dimensionless quantities

We should be able to exponentiate dimensionless quantitites. For a motivated example:
```julia
using DynamicQuantities.Constants: a_0

R10(r) = 2 / a_0^(3/2) * exp(-r/a_0)
R10(3 * a_0)
```
And a minimal case:
```julia
using DynamicQuantities
exp(1u"m" / 1u"m")
```
With error:
```julia
ERROR: MethodError: no method matching exp(::Quantity{Float64, Dimensions{DynamicQuantities.FixedRational{Int32, 25200}}})
```
Happy to put in a PR, although it may be a few days due to schoolwork.