Add angular units

[MilesCranmer]

Fixes #28

@mikeingold since you have interest in using this, what do you think?

Here I am using the SI definition that 1 rad = 1. So to have "rad" actually show up in quantities you would need to use symbolic units, like the following:

julia> θ = 10.2us"deg"
10.2 deg

julia> r = 5us"km"
5.0 km

julia> θ * r |> uexpand
890.117918517108 m

which I think seems okay?

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Benchmark Results

	main	1424a94fe06ce9...	main/1424a94fe06ce9...
Quantity/creation/Quantity(x)	3.41 ± 0.01 ns	3.71 ± 0.011 ns	0.919
Quantity/creation/Quantity(x, length=y)	3.11 ± 0.01 ns	3.42 ± 0.01 ns	0.909
Quantity/with_numbers/*real	3.11 ± 0.01 ns	3.1 ± 0.01 ns	1
Quantity/with_numbers/^int	8.05 ± 2.2 ns	8.37 ± 2.2 ns	0.963
Quantity/with_numbers/^int * real	8.68 ± 2.2 ns	8.05 ± 1.8 ns	1.08
Quantity/with_quantity/+y	4.04 ± 0.001 ns	4.04 ± 0.001 ns	1
Quantity/with_quantity//y	3.42 ± 0.01 ns	3.11 ± 0.001 ns	1.1
Quantity/with_self/dimension	3.1 ± 0.01 ns	2.79 ± 0.009 ns	1.11
Quantity/with_self/inv	3.11 ± 0.001 ns	3.11 ± 0.001 ns	1
Quantity/with_self/ustrip	2.79 ± 0.01 ns	2.79 ± 0.01 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.145 ± 0.00075 ms	0.15 ± 0.0008 ms	0.967
QuantityArray/broadcasting/multi_normal_array	0.0529 ± 0.00028 ms	0.0498 ± 0.00029 ms	1.06
QuantityArray/broadcasting/multi_quantity_array	0.155 ± 0.00066 ms	0.155 ± 0.00059 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	25 ± 3 μs	24.6 ± 2.8 μs	1.02
QuantityArray/broadcasting/x^2_normal_array	4.52 ± 1.1 μs	4.24 ± 1.1 μs	1.07
QuantityArray/broadcasting/x^2_quantity_array	6.98 ± 0.28 μs	6.9 ± 0.28 μs	1.01
QuantityArray/broadcasting/x^4_array_of_quantities	0.0787 ± 0.00067 ms	0.0787 ± 0.00057 ms	1
QuantityArray/broadcasting/x^4_normal_array	0.0467 ± 0.00019 ms	0.0497 ± 0.00014 ms	0.94
QuantityArray/broadcasting/x^4_quantity_array	0.05 ± 0.003 ms	0.0499 ± 0.00016 ms	1
time_to_load	0.127 ± 0.00052 s	0.127 ± 0.00034 s	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[mikeingold]

I just added this branch directly into Julia and (at a glance) it seems to behave like I'd expect ,e.g.:

Leλ_units_preferred = us"mW/(m^2*sr*μm)"

val = 1.0us"W/(cm^2*sr*nm)"

2val |> uconvert(Leλ_units_preferred)
# 1.9999999999999996e10 m⁻² μm⁻¹ mW sr⁻¹

[MilesCranmer]

Cool. Thanks! And yeah it looks like Unitful does this the same way so all is good.