make the naming of anemoi.models.* consistent

[floriankrb]

Is your feature request related to a problem? Please describe.

We currently have:

commands
data_indices
distributed
interface
layers
models
preprocessing

Describe the solution you'd like

We mix singular/plurals, -ing forms. I suggest to make it consistent.

commands
data_indices -> indices
distributed
interface -> interfaces
layers
models
preprocessing -> processors

We can easily make the change backward compatible.

Describe alternatives you've considered

No response

Additional context

No response

Organisation

No response