allow simulations with multiple transducers

[neurodream]

The use of multiple (bowl-shaped) transducers can be beneficial w.r.t. focal size and skull heating (see e.g. here or here).

The PRESTUS code seems quite optimized for a one transducer setup, but I think allowing for N >= 1 transducers would be helpful in general. I am currently working on an implementation (i.e. generalizing the code for multiple transducers) but I want to make sure it does not make the code and the outputs too chaotic.

Here is the basic idea of implementation:

• on the configs (i.e. user input) side:
  • instead of a struct transducer, call it transducers, with one item per transducer
  • rather than one "global" struct, move focus_pos_t1_grid within a "transducer" item, to allow for different transducers targeting different regions (if needed)
• within PRESTUS:
  • add all transducer elements to the same kwave source grid
• outputs:
  • have separate (debug) image files for each transducer, and add transducer IDs to the filenames
  • adding columns for each transducer-target in the overview output table

What this would not (yet) do:

• allow for different stimulation parameters per transducer

I will add and explain the specific changes in the code from my implementation in due time.