Add requirements for each component type in src/models

[Alicegaz]

It is not obvious how to discriminate between nn blocks in different subdirectories of src/models. And it is better to add readme and base classes for each component (block, brick, module, etc) listing all the requirements (what it accepts, what it returns, what it is allowed and supposed to do with the inputs).

[VladVin]

Indeed, we need a documentation for this