Add wavelength option to specify sources and monitors

[e-g-melo]

Background

In GUI, we allow users to select between wavelength or frequency to define sources and monitors.

Currently, if users select wavelength, they need to provide the minimum and maximum wavelengths and can adjust the bandwidth using the "Customize source bandwidth" parameter.

Or they can input the frequency and bandwidth as below.

The relation between wavelength and frequency is defined as:

freq0 = (td.C_0 / lamb_min + td.C_0 / lamb_max) / 2
fwidth = customize_source_bandwidth * (td.C_0 / lamb_min - td.C_0 / lamb_max)

For monitors the user can also select between wavelength and frequency, and the conversion is done using freq = td.C_0 / lamb.

Requirement

Some GUI users have requested to set the source using a central wavelength and a wavelength bandwidth, and we have Python users also asking to set sources and monitors using wavelength.

So, we could have the following options to set sources:

1. freq0 and fwidth
2. lamb0 and lamb_width
3. freq_min, freq_max, and customize_source_bandwidth
4. lamb_min, lamb_max, and customize_source_bandwidth

For monitors, the user could assign a list of frequencies or a list of wavelengths.

In GUI, it would be helpful if we could have a method to return freq0, fwidth, lamb0, lamb_width, freq_min, freq_max, lamb_min, and lamb_max. So, if the user sets, for instance, freq_min and freq_max, we can easily obtain the other values to show in UI when they switch between the available input options.

The customize_source_bandwidth parameter is not that intuitive for users, if we have a better approach to adjust the source bandwidth when the users select min/max that would be nice.