Add Radio Astronomy plugin

[srcejon]

This patch adds a Radio Astronomy plugin. It supports:

• A spectrometer for displaying time averaged spectra.
• A radiometer for displaying time averaged continuum measurements (total power).
• Calibration to enable measurements to be displayed as noise temperatures (K), power (dBm/Watts) and spectral flux density (Jy).
• Utilities are included for estimation and calculation of noise temperature components (Tsys, Trx, Tgal, Tatm, Tsky, Tsp) and sensitivity (sigma Tsys and sigma Sv).
• Spectra can be displayed against frequency and velocity (with a configurable reference spectral line), with the velocity adjusted to topocentric, Solar System barycentric or the Local Standard of Rest (LSR) reference frames.
• Calculation and plotting of radial and Galactocentric distance to HI clouds, based on spectral peaks.
• Position of HI clouds can be sent to Star Tracker plugin for visualisation on the Galactic line-of-sight image and created in to an animation mapping out the Milky Way's spiral arms.
• A Gaussian fitting tool in the spectrometer for HI cloud kinetic temperature and column density estimation.
• A Gaussian fitting tool in the radiometer to enable antenna HPBW measurement from Solar drift-scans.
• Ability to record and plot real-time surface air temperature and other sensor measurements (component voltages / temperatures) alongside radiometer measurements.
• Ability to export charts to animated .png files and static image files.
• Reference spectra from the LAB (Leiden/Argentine/Bonn) Galactic HI survey can be automatically downloaded and plotted for comparison against user measurements.
• 2D sweeps can be made and plotted in different coordinate systems (Az/El, Galactic, offsets around a target and drift scans).
• All spectra are held in memory and can be scrolled through.
• Data can be saved and loaded from .csv files.
• Hardware for calibration (E.g. RF switches) can be automatically controlled.

It also includes some updates to the Star Tracker feature:

• Plot Sun and Moon on sky temperature chart.
• Plot markers on Galactic line-of-sight chart.
• Create animations from Galactic line-of-sight chart.
• Allow weather at antenna location to be downloaded from openweathermap.org
• Allow target to be entered as Galactic longitude / latitude.
• Add azimuth and elevation offsets to support scans around targets.
• Add S7, S8 and S9 targets.
• Refactor some code from GUI to main plugin, so computed values can be used in other plugins.

As well as a fix for #1010 in the GS-232 Controller.

In 05fce63 I've added support for messages pipes from Features to Channels. You may want to review this to see if it can be done more nicely / tidied up. One particular issue is the Feature inherits from QObject, but ChannelAPI doesn't, so the passing of messages is a little different between the two.