Add pico and nano prefixes for gram

[MilesCranmer]

Fixes #69 by @jkrumbiegel

[github-actions]

Benchmark Results

	main	f379733...	t[main]/t[f379733...]
Quantity/creation/Quantity(x)	3.1 ± 0.1 ns	3.1 ± 0.1 ns	1
Quantity/creation/Quantity(x, length=y)	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_numbers/*real	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_numbers/^int	11.4 ± 3.7 ns	11.4 ± 3.6 ns	1
Quantity/with_numbers/^int * real	12.1 ± 4 ns	11.9 ± 4 ns	1.02
Quantity/with_quantity/+y	7.7 ± 0.1 ns	7.7 ± 0.1 ns	1
Quantity/with_quantity//y	4 ± 0 ns	4 ± 0 ns	1
Quantity/with_self/dimension	1.7 ± 0 ns	1.7 ± 0 ns	1
Quantity/with_self/inv	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_self/ustrip	1.7 ± 0 ns	2 ± 0 ns	0.85
QuantityArray/broadcasting/multi_array_of_quantities	0.234 ± 0.031 ms	0.234 ± 0.026 ms	1
QuantityArray/broadcasting/multi_normal_array	0.0821 ± 0.0046 ms	0.0821 ± 0.0042 ms	1
QuantityArray/broadcasting/multi_quantity_array	0.255 ± 0.0017 ms	0.255 ± 0.0015 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	0.063 ± 0.014 ms	0.0631 ± 0.015 ms	0.998
QuantityArray/broadcasting/x^2_normal_array	12.5 ± 4 μs	12 ± 3.9 μs	1.04
QuantityArray/broadcasting/x^2_quantity_array	13.3 ± 1.3 μs	13.4 ± 1.3 μs	0.993
QuantityArray/broadcasting/x^4_array_of_quantities	0.148 ± 0.0061 ms	0.149 ± 0.0061 ms	0.999
QuantityArray/broadcasting/x^4_normal_array	0.0697 ± 0.0013 ms	0.0696 ± 0.0012 ms	1
QuantityArray/broadcasting/x^4_quantity_array	0.0986 ± 0.0095 ms	0.0956 ± 0.011 ms	1.03
time_to_load	0.174 ± 0.0023 s	0.176 ± 0.0033 s	0.987

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).