add wavelength dependence to thin lenses

[bruhwiler]

Chromatic effects are significant in laser amplifiers and must be included.
The predominant effect can be captured by including wavelength dependence in thin lenses.

The thin lens class should have a data member specifying the central wavelength, as well as the focal length.
The wavefront propagator class should query this wavelength and calculate a delta_lambda from the laser pulse slice.

A reasonable wavelength dependence of the focal length should be implemented and applied.
If the wavelength difference is too large, then an exception should be thrown.

@bnash -- please take the lead on identifying an appropriate algorithm for wavelengths around 800 nm and (presumably) glass lenses. Work with @gurhar1133 to get the algorithm implemented.

This is related to issue #39.

[bruhwiler]

This is an important feature, but development will have to wait until new resources have been identified.