Multi-domain optimization: unique n_particles for each domain

[annehaley]

As of #336, the value sent in to the optimization step for "number_of_particles" is the same value repeated for every domain in each subject. (e.g. for a two-domain case, the value would be something like "32 32".)

The user should be able to specify a unique number of particles for each available domain. This would require a variable number of inputs on the form.