Created terrain information functions for Python3 module (getTERR and sendPOST)

[axkralj990]

I created two new functions (getTERR and sendPOST) for reading terrain information with the Python3 module. Both functions are based on the functions with the same name for the C module (#204 and #205).

• getTERR allows users to read terrain information (lat, lng, elevation, normals (nx, ny, nz), wet/dry, status) by sending a TERR UDP message.
• sendPOST sets the position of the specified aircraft and returns terrain information by sending a POST UDP message (this is a combination of sendPOSI and getTERR, as explained in Created sendPOST function that combines functions of sendPOSI and getTERR #205).

[axkralj990]

In the most recent commit (da8a7ef), I fixed the UDP terrain response unpacking problem. Initially, I referred to the response message structure described in the spreadsheet posted in the comments of #186, and I ignored the last 3 4-byte elements of the message. Both functions now return all the elements contained in the response ([Lat, Lon, Alt, Nx, Ny, Nz, Vx, Vy, Vz, wet, result]).