Add ability to remove box fluctuations when unwrapping NPT trajectories.

[drroe]

Version 6.19.1.

Adds new command for calculating average unit cell:

  [help avgbox]
	[<setname>] [out <file>]
  Calculate average box.

This in turn can be used with the unwrap command for removing box fluctuations from NPT trajectories when unwrapping via the new avgucell keyword. Adds tests for both, updates manual (including an example and advising people to do this when calculating diffusion for NPT trajectories).

[drroe]

Usual pytraj jenkins failure.