`copy_number` of EFLR items

[the-mysh]

Each EFLRItem has an attribute copy_number. It is meant to distinguish between two EFLR items of the same type and name.

The copy_number is computed automatically at the level of an EFLRItem by checking how many EFLRItem instances of the given name are already defined in the parent EFLRSet and subtracting 1 (so for the first ELFRItem of the given type and name, copy_number is 0).

However, this is only checked at the level of the parent EFLRSet of the given EFLRItem. In most cases this is sufficient, because normally a single EFLRSet of a given type is defined in a file, but it is possible to define more (see this issue). In case multiple EFLRSets are added to the same file, it is likely that multiple EFLRItem instances will share the same type, name, and copy_number, making them indistinguishable in DLIS references. As a result, references might point to the wrong items (resulting i.e. in a FrameItem displaying the wrong channel data).