Fix gadget inheritances inconsistencies

[AntoineRondelet]

So far, we have followed a "public inheritance" approach when inheriting from the libsnark::gadget class and set the right scopes on the class' attributes and methods. However, the aggregator_gadget inherits privately from gadget which contrasts with the approach just mentioned (following public inheritance), see: https://clearmatics.github.io/zecale/inherits.html for illustration.
This inconsistency needs to either:

• Be fixed, or
• Be documented (if justified)