LengthField multiplier does not support referencing other fields

[martinpelikan]

The protocol I'm working with includes two pieces of data in individual fields needed to determine the total payload length that I would have in a FieldArray's length_provider:

1. The substructure length
2. The number of substructures

From the docs it looks like the multiplier in LengthField is what number 2) is above. The current implementation appears to only support integers however.