Add Molar units (M)

[TorkelE]

Not 100% sure what is required to implement a new unit, but my impression is that this is the approach? Please tell me if I should add something.

[github-actions]

Benchmark Results

	main	fd7ef9b...	main/fd7ef9bc5861ba...
Quantity/creation/Quantity(x)	3.72 ± 0.01 ns	2.79 ± 0 ns	1.33
Quantity/creation/Quantity(x, length=y)	3.41 ± 0.01 ns	3.42 ± 0.01 ns	0.997
Quantity/with_numbers/*real	3.11 ± 0.01 ns	3.11 ± 0.01 ns	1
Quantity/with_numbers/^int	8.05 ± 1.8 ns	8.37 ± 2.2 ns	0.963
Quantity/with_numbers/^int * real	8.36 ± 1.9 ns	8.05 ± 1.8 ns	1.04
Quantity/with_quantity/+y	4.04 ± 0.01 ns	4.04 ± 0.001 ns	1
Quantity/with_quantity//y	3.11 ± 0.001 ns	3.11 ± 0.001 ns	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	4.02 ± 0.92 ns	0.773
Quantity/with_self/ustrip	2.79 ± 0.009 ns	2.79 ± 0.01 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.145 ± 0.00056 ms	0.147 ± 0.00045 ms	0.985
QuantityArray/broadcasting/multi_normal_array	0.0528 ± 0.00015 ms	0.0498 ± 0.00015 ms	1.06
QuantityArray/broadcasting/multi_quantity_array	0.158 ± 0.00038 ms	0.155 ± 0.00063 ms	1.02
QuantityArray/broadcasting/x^2_array_of_quantities	25.5 ± 2.1 μs	25.6 ± 2.2 μs	0.998
QuantityArray/broadcasting/x^2_normal_array	4.61 ± 1.1 μs	4.39 ± 1.2 μs	1.05
QuantityArray/broadcasting/x^2_quantity_array	7.06 ± 0.38 μs	7 ± 0.33 μs	1.01
QuantityArray/broadcasting/x^4_array_of_quantities	0.0786 ± 0.00049 ms	0.0786 ± 0.00047 ms	1
QuantityArray/broadcasting/x^4_normal_array	0.0498 ± 0.00017 ms	0.0497 ± 0.00015 ms	1
QuantityArray/broadcasting/x^4_quantity_array	0.05 ± 0.00015 ms	0.0499 ± 0.00014 ms	1
time_to_load	0.127 ± 0.0002 s	0.128 ± 0.00043 s	0.996

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).