New Particulator creation flow

[abulenok]

reasons:

• avoid modifying env and backend by Builder::init(..., env, backend)

user-perspective:

• backend = Backend()
• Env(backend)
• create list of dynamics
• create dictionary of attributes (old init_attributes logic should go )
• create list of products
• Particulator(env, dynamics, attributes, products)

steps:

• [ ] remove Builder class?
• [ ] release 3.0

[slayoo]

idea for decorator name: @pysdm.product_prototype (with a method instantiate(backend) returning a Product instance)

[github-actions[bot]]

Stale issue message

[slayoo]

it is probably Backend(env) what we need rather than Env(backend)! The reason is that backend should leverage the information on dimensionality of the environment and, e.g., avoid multi-threading for cell-wise operations if there is just one cell. Similarly, the displacement code will get much simpler and will no longer need repeated if-else statements checking the dimensionality at runtime on every call.

[slayoo]

... or Backend(n_dims: int) ?

[slayoo]

idea for decorator name: @pysdm.product_prototype (with a method instantiate(backend) returning a Product instance)

Done in #1370

[slayoo]

dynamic-reuse issue addressed in #1372 with a new @register_dynamic decorator

[slayoo]

environment reuse issue addressed in #1373 with a new @register_environment decorator