Specify tile class in RPU Config

[diego-plan9]

Related issues

Description

Update the BaseTile in order to retrieve the tile class it needs to instantiate from the RPUConfig (via a new tile_class attribute defined on those), decoupling tiles from analog layers (albeit not fully). With this change (which needed #213), a user should have an easier way to use a custom tile via defining a RPUConfig that uses the tile - for example (from the test):

class CustomAnalogTile(AnalogTile):
    """Helper tile for ``CustomTileTest``."""
    ...

class CustomRPUConfig(SingleRPUConfig):
    """Helper rpu config for ``CustomTileTest``."""
    tile_class = CustomAnalogTile
    ...

my_layer = AnalogLinear(..., rpu_config=CustomRPUConfig(...))

Details