introduce a unique identifier (uoid) for each odml object

[msuzen]

following discussion from #93,setting uoid (unique object id) when an object is created. Use case could be that this can be used in reference or in setting parent attribute. Maybe a SHA-1 string.

[lzehl]

There is currently a Section attribute called id... not sure if this was attended to be used for something like this?

[msuzen]

c.f. #45 too.

[msuzen]

@lzehl Maybe uuid is a better name.

[mpsonntag]

Has been resolved since #121