Add uconvert method for QuantityArray

[MilesCranmer]

cc @gaurav-arya

(For what it's worth, it seems like using uconvert(us"km").(array) works, but it relies on compiler inlining a lot of stuff to be as efficient. Also, expand_units has a dedicated method, so I think it makes sense to do it here as well.)

[github-actions]

Benchmark Results

	main	3333cd9...	t[main]/t[3333cd9...]
Quantity/creation/Quantity(x)	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/creation/Quantity(x, length=y)	3.7 ± 0 ns	3.7 ± 0 ns	1
Quantity/with_numbers/*real	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_numbers/^int	11.5 ± 4.1 ns	11.5 ± 4.1 ns	1
Quantity/with_numbers/^int * real	11.8 ± 4 ns	12 ± 4 ns	0.983
Quantity/with_quantity/+y	6.8 ± 0.1 ns	6.8 ± 0.1 ns	1
Quantity/with_quantity//y	3.7 ± 0.001 ns	3.7 ± 0.001 ns	1
Quantity/with_self/dimension	1.7 ± 0 ns	1.7 ± 0.001 ns	1
Quantity/with_self/inv	3.4 ± 0 ns	3.4 ± 0.001 ns	1
Quantity/with_self/ustrip	1.7 ± 0 ns	1.7 ± 0 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.225 ± 0.007 ms	0.226 ± 0.023 ms	0.999
QuantityArray/broadcasting/multi_normal_array	0.0764 ± 0.0022 ms	0.0761 ± 0.0021 ms	1
QuantityArray/broadcasting/multi_quantity_array	0.255 ± 0.0014 ms	0.254 ± 0.0013 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	0.0426 ± 0.0045 ms	0.0414 ± 0.0028 ms	1.03
QuantityArray/broadcasting/x^2_normal_array	8.7 ± 1.5 μs	9 ± 1.7 μs	0.967
QuantityArray/broadcasting/x^2_quantity_array	10.2 ± 0.8 μs	10.2 ± 1.4 μs	1
QuantityArray/broadcasting/x^4_array_of_quantities	0.136 ± 0.0074 ms	0.135 ± 0.0058 ms	1.01
QuantityArray/broadcasting/x^4_normal_array	0.0688 ± 0.0014 ms	0.069 ± 0.0013 ms	0.997
QuantityArray/broadcasting/x^4_quantity_array	0.104 ± 0.0018 ms	0.103 ± 0.0021 ms	1.01
time_to_load	0.165 ± 0.0031 s	0.165 ± 0.00031 s	0.998

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).

[gaurav-arya]

Looks good to me. One nit: the @assert isone(... is missing for the array method.

[MilesCranmer]

Good catch! Added.