Inject new particles based on location

[boriskaus]

It would be great to have a possibility to inject particles in a certain location in the grid, and not only when the number of particles are below a certain threshold.
The use case for that is injecting magma-filled sills/dikes in a model setup, as for example done in MagmaThermoKinematics. Typically the injected dikes/sills are quite thin compared to the grid cell. Yet, we still want to take their temperature into account as this heats up the crust, which would require particles to be added within the dike.