Fix plotting of individual universe levels.

[pshriwise]

Attempting to plot a specific universe level is risky right now after the changes in #1201. If the universe level is higher than the number of coordinates for the particle at a given pixel location in the image, it's possible that an invalid access in the particle coordinates can occur, resulting in a segfault. The relevant chunk of code is here:

openmc/src/plot.cpp

Lines 63 to 71 in 828e0aa

	void
	PropertyData::set_value(size_t y, size_t x, const Particle& p, int level) {
	Cell* c = model::cells[p.coord_[level].cell].get();
	data_(y,x,0) = (p.sqrtkT_ * p.sqrtkT_) / K_BOLTZMANN;
	if (c->type_ != Fill::UNIVERSE && p.material_ != MATERIAL_VOID) {
	Material* m = model::materials[p.material_].get();
	data_(y,x,1) = m->density_gpcc_;
	}
	}

This PR corrects that by setting any pixel color to NOT_FOUND when the coordinate level of the particle is lower than the requested universe level. For density and temperature plots, the particle's coordinate level is always used.

This also exposes the model::n_coord_level variable in the CAPI and the openmc.lib module to allow our plotting application to correctly populate the number of available universe levels.

[paulromano]

Thanks @pshriwise!