Support ExperimentalBoutonDensity ExperimentalNeuronDensity ExperimentalSynapsesPerConnection

[GianlucaFicarelli]

Description

Add support for entities:

• ExperimentalBoutonDensity
• ExperimentalNeuronDensity
• ExperimentalSynapsesPerConnection

Technical implementation & specs

These entities should match the schemas defined in entitycore.

Additional context

Needed for openbraininstitute/entitycore#334

Acceptance criteria

• entitysdk can create, read and update ExperimentalBoutonDensity
• entitysdk can create, read and update ExperimentalNeuronDensity
• entitysdk can create, read and update ExperimentalSynapsesPerConnection

[evakennyobi]

This needs a priority & time estimate please @GianlucaFicarelli