Correct implementation of C

[FantasyVR]

This computation of C as following is not consistent with this line.

taichi_mpm/mls-mpm88-explained.cpp

Line 171 in 3bb90fb

p.C += 4 * inv_dx * Mat::outer_product(weight * grid_v, dpos);

The above line is lack of one 1/dx according to the last page of this slide.

[RobertBiehl]

@FantasyVR I think the reason is, that dos is already divided by dx. Bit hard to read, but it seems to be correct.

[FantasyVR]

I think it should be
p.C += 4 * inv_dx * inv_dx * Mat::outer_product(weight * grid_v, dpos);

@FantasyVR I think the reason is, that dos is already divided by dx. Bit hard to read, but it seems to be correct.