Feature request: dimension helper functions?

Hi Miles, just following up on my evil master plan for JuliaAstro, would you be open to a PR adding some convenience `is` functions?

I'm not sure if this has been discussed somewhere already, but we're basically looking for a convenient way to dispatch on dimensions, e.g.,

```julia-repl
julia> using DynamicQuantities

julia> x = 3u"m"
3.0 m

julia> v = 3u"m/s"
3.0 m s⁻¹

julia> function do_stuff(q::UnionAbstractQuantity)
if islength(q)
#
elseif isspeed(q)
#
# ...
else
end
end
```

I see that this could be done by factoring out some of the [dimension display logic here](https://github.com/JuliaPhysics/DynamicQuantities.jl/blob/dfba00cb164b44ef610e1c677a683a4e57a6c524/src/utils.jl#L328-L339), to something like:

```julia-repl
julia> function islength(q::UnionAbstractQuantity)
d = dimension(q)
dd = filter(k -> !iszero(d[k]), keys(d))
return dd == (:length,) && d.length == 1
end
islength (generic function with 1 method)

julia> function isspeed(q::UnionAbstractQuantity)
d = dimension(q)
dd = filter(k -> !iszero(d[k]), keys(d))
return dd == (:length, :time) && d.length == 1 && d.time == -1
end
isspeed (generic function with 1 method)

julia> islength(3u"m")
true

julia> isspeed(3u"m/s")
true
```

Alternatively, would this make sense as a trait that could be dispatched on at the type level, or would this run counter to the dynamic runtime design here?