Make it possible to create a bag without moving the payload

[RKrahl]

When creating a new bag, make_bag() takes the bag directory as argument. It assumes the current content of this bag directory to be the payload. It creates a new subdirectory data and moves all content of the bag directory into this new subdirectory.

It should be possible to create a bag without moving the payload, given that it is already in a suitable data subdirectory. This is particularly needed when only read access to the payload can be granted.

I submitted a pull request #67 to add such an option, but I got no response. Maybe it has not been noticed. So I submit this Issue to draw your attention.

[acdha]

Closing as duplicate of #67