Harmonise Fiber and Sheaf interfaces

[opeltre]

Fibers and sheaves both represent index ranges - a fiber is the same as a sheaf over a point.

Harmonising both interfaces will allow to have sheaves whose fibers are also sheaves.

This is particularly useful to define restrictions, quotients, fibrations...

(continues #12)

[opeltre]

To do this, one should first define the class method Sheaf.join(sheaves) (see Fiber.join here) to compute offsets based on each sheaf's size.

Then have a look at methods such as Sheaf.index / Fiber.index