Add support for nested ULOG fields within ULOG generator

[filipembedded]

For now, it's not possible for Ulog generator to support nested .yaml fields. It could be handy if there is a case where there are multiple fields that have subfields that are named the same.
Example:

name: some_sensor
description: Contains sensor data - x, y and z
field1:
  - name: x
    type: float
  - name: y
    type: float
  - name: z
    type: float
field2:
  - name: x
    type: float
  - name: y
    type: float
  - name: z
    type: float