Add option to specify camera axis direction

[marius-ne]

Hello!

I have added a feature that allows the user to specify the direction that the camera (boresight) axis is pointing to, either in the positive or negative z directions.

The original code had -z by default which I found surprising because in my line of work, +z is the norm. So I'm adding this for other people who might have the same experience.

I am open for suggestions and if you agree @xt4d I will add some docstrings to the methods to explain the new option along with the other, pre-existing arguments. I would also like to add being able to specify -/+ x/y axes for the camera axis in the future.

Note: One has to be careful when specifying -z as an option on the command line, the string needs to be properly escaped and passed to the option like this: --camera_axis="-z"

Thanks!

PS: Here an illustration that shows the new feature:

[marius-ne]

Resolves #4