Add frequency plan

#11 (closed) adds laser locking, but frequency offsets are still static and controlled by the option laser_offsets.

We should add the possibility for the user to either

• provide a set of constant beatnote frequencies or offsets (to be decided)
• provide a frequency plan file generated by LISA Frequency Plan, from which we can read the locking beatnote frequencies

To be consistent, we should probably allow the user to specify constant locking beatnote frequencies. So that we can initialize the frequency plan in the __init__() method, in order to read and interpolate (if needed) the locking beatnote frequencies. They can then be used later on in the simulation.

How should we represent the dictionary of locking beatnote?