Add the classes for EOM mode configuration

[HGSilveri]

• Convert channels.py module into a module directory
• Creates the classes for configuring operation in the EOM mode. Their usage by Sequence will come in a second PR, but is already done on branch eom-seq.

[madagra]

I have some minor comments (mainly due to a lack of context here which can be covered offline).

The main point I do not get from this PR is how the EOM modulation is used in practice in a simulation. From what I understand now, the only things which are actually used in the Channel.modulate() method from the EOM class are the values of modulation bandwidth and rise time. I guess this will be part of the follow-up PR, right?

[madagra]

I have a final comment on the unit tests, but it looks good to me after that. It might be useful to add in the documentation some succinct information on how the detuning offset and light shift is computed (with references on where it comes from).

[madagra]

Thanks, it looks good to me.