add option for setting time zone

[nwolek]

Filenames currently use the date and time in UTC timezone. When converting this into text for the spectrogram movies, the script will convert it into the current time zone of your machine. Here is the relevant line of code:

https://github.com/nwolek/audiomoth-scripts/blob/master/make-spectrogram-movie.sh#L34

This is a problem if your AudioMoth recordings were recorded in another time zone. As a workaround, you can simply change the time zone in your system preferences to match where the recordings were made.

As an enhancement, it would be nice to have an option for setting the timezone of the recordings.