Add force_injection!

[albert-de-montserrat]

This PR is addresses #186 and adds the function ``force_injection!` to inject a given set of new particles in a desired subset of cells.

Usage

• Define the new p_new particles object. In the bi-dimensional case p_new::Array{Union{Point{2, Float64}, SVector{2, Float64}} 3} is of size $nx \times ny \times np$ where $np$ is the number of new particles per given cell. Indices of p_new where there is no new particle must be NaN.

• Use case (1) Inject only new particle coordinates:

force_injection!(particles::Particles, p_new)

• Use case (2) Inject new particles and specific values for given fields defined at the particles. For example, if we want all new particles to have a brand new phase we can do as:

phases, = init_cell_arrays(particles, Val(1))
phases.data .= 1
phases.data[:, 16:end, :] .= 2

force_injection!(particles, p_new, (phases,), (3e0,))