Add ability to calcualte weighted volume vectors and VTMs

[dgaylo]

Desired Feature

Currently, ELA_OutputWriteVTM() outputs the (discretized version of the) VTM

$$ q_{ml} = \iiint c_m^{n+1}\left(\vec{x}\right) \; c_l^{n}\left(\vec{x}\right) \mathrm{d}V \;. $$

It would be useful if there was a way to output the VTM weighted by some $f\left(\vec{x}\right)$, i.e.,

$$ r_{ml} = \iiint f\left(\vec{x}\right) \; c_m^{n+1}\left(\vec{x}\right) \; c_l^{n}\left(\vec{x}\right) \mathrm{d}V \;. $$

For example, setting $f\left(\vec{x}\right)=x$ would allow one to calculate the centroid $x_{ml} = r_{ml}/q_{ml}$ of the volume described by $q_{ml}$.

[dgaylo]

Currently, this could be achieved by a user by modifying the cell volume dV passed to ELA_OutputWriteVTM(..., dV, ...):

$$ \Delta \Omega_{ijk}' = \Delta \Omega_{ijk} f_{ijk} $$