Inconsistent particle discretization

[d-cogswell]

Mpet takes a particle radius and a computational cell width as input, but there is no guarantee that the number of cells (radius/discretization) will be an integer. When it's not an integer mpet rounds it to an integer, which slightly changes the radius of the particle.

This choice was probably made so the phase-field models would have enough resolution for the phase boundaries. We may want to create an option for a desired number of computational cells, or at least print a warning when the number of cells is not an integer.