Implement the plasma critical density in the formulary

[pheuer]

The critical plasma density for a given frequency of light is the electron density at which the laser frequency equals the electron plasma frequency. This is an important quantity because light will not propagate above the critical density. As a result, laser light is deposited around this density in laser-solid interactions and optical plasma diagnostics only work below it.

It would be great if this formula was added to the PlasmaPy formulary.