Change computation of cube size - taking haov into account

* support also partial cubemap/fallback images: backgroundColor for missing face(s)

* extend multires/generate.py to cylindrical input and partial panoramas

* fixed a few typos in new help strings of generate.py

* Missing cubemap faces now need to be defined as null in the configuration

* made generate.py strict again on existing output directory, printing descriptive error message

* replaced unsafe eval by ast.literal_eval in generate.py

* a couple of minor improvements regarding 'undefined' in libpannellum.js

* made generate.py allow existing output directory in debug mode

for convenience, no need to delete output directory every time while testing

Extra dot in front of paths like 'path' and 'fallbackPath' are not necessary and may cause unexpected errors.

Replaces the os.system call for nona with a subprocess.check_call call
to get informed about errors when calling nona instead of silently
ignoring them before failing with missing files.

Use a formatter for argparse to print default values instead of doing
this manually.

Checks for a nona executable and adds a command line argument to specify
an executable. The argument is required from the user in case nona
cannot be found on the path.

Encode the explicit requirement on python 3 in the shebang line for
script usage.