Add missing method for power of quantity to other quantity

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Benchmark Results

	main	1f2c1a5f0190af...	t[main]/t[1f2c1a5f0190af...]
Quantity/creation/Quantity(x)	2.79 ± 0.01 ns	2.79 ± 0.01 ns	1
Quantity/creation/Quantity(x, length=y)	3.11 ± 0.01 ns	3.11 ± 0.91 ns	1
Quantity/with_numbers/*real	3.1 ± 0.01 ns	4.02 ± 0.92 ns	0.771
Quantity/with_numbers/^int	8.05 ± 2.2 ns	8.05 ± 2.2 ns	1
Quantity/with_numbers/^int * real	8.05 ± 2.2 ns	8.05 ± 2.2 ns	1
Quantity/with_quantity/+y	5.27 ± 0.01 ns	5.27 ± 0.01 ns	1
Quantity/with_quantity//y	3.41 ± 0.01 ns	3.41 ± 0.01 ns	1
Quantity/with_self/dimension	1.55 ± 0.01 ns	1.55 ± 0.01 ns	1
Quantity/with_self/inv	3.11 ± 0.01 ns	3.11 ± 0.01 ns	1
Quantity/with_self/ustrip	1.55 ± 0.01 ns	1.55 ± 0.01 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.147 ± 0.14 ms	0.147 ± 0.14 ms	0.998
QuantityArray/broadcasting/multi_normal_array	0.0621 ± 0.0091 ms	0.0529 ± 0.00021 ms	1.17
QuantityArray/broadcasting/multi_quantity_array	0.158 ± 0.00069 ms	0.158 ± 0.00075 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	25.1 ± 2 μs	25.6 ± 2 μs	0.978
QuantityArray/broadcasting/x^2_normal_array	4.49 ± 1 μs	4.43 ± 0.99 μs	1.01
QuantityArray/broadcasting/x^2_quantity_array	6.04 ± 0.38 μs	6.12 ± 0.32 μs	0.987
QuantityArray/broadcasting/x^4_array_of_quantities	0.0785 ± 0.00043 ms	0.0786 ± 0.00051 ms	0.998
QuantityArray/broadcasting/x^4_normal_array	0.0498 ± 0.00016 ms	0.059 ± 0.00017 ms	0.844
QuantityArray/broadcasting/x^4_quantity_array	0.0592 ± 0.00016 ms	0.0592 ± 0.00016 ms	1
time_to_load	0.241 ± 0.0015 s	0.238 ± 0.001 s	1.01

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).