FIX: Deserialization of sized variable items

[HGSilveri]

We were only supporting the deserialization of unsized variable items (e.g. single values obtained through var[0]), although we also supported sized variable items through slices (e.g. var[1:5:2]).

The abstract representation allowed for serialization of these variable items but then deserialization failed. The easiest way to fix this was actually to add support for indexing variables with a list of indices (e.g. var[[0, 1, 3]]), so this hotfix actually introduces a small enhancement.

• Fixes deserialization from the abstract representation of sized variable items
• Adds option to get a sized VariableItem by indexing a Variable with a list of indices
• Supports __len__ for sized VariableItem instances

[a-corni]

Looks good to me !