Class to transparently deal with the conversion between filters, wavelength,
frequency and other methods of specifying a location in the electro-magentic
spectrum.

Astro::WaveBand tries to determine the natural form of the numbers such that a
request for a summary of the object when it contains 2.2 microns would return
the filter name but would return the wavelength if it was not a standard filter.
In ambiguous cases an instrument name is required to decide what to return. In
really ambiguous cases the user can specify the unit in which to display the
numbers on stringification.

Used mainly as a way of storing a single number in a database table but using
logic to determine the number that an observer is most likely to understand.

Numerical comparison operators can be used to compare two Astro::WaveBand
objects. When checking equality, the "natural" and "instrument" methods are
used, so if two Astro::WaveBand objects return the same value from those
methods, they are considered to be equal. When checking other comparisons such
as greater than, the wavelength is used.