Fix virtual mass equation formatting (!160)

The following input is defined using the prefix ``mfix.drag``:

.. |VirtualMass_Eq| replace:: :math:`\mathbf{F}_p,vm = -C_p,vm \rho_f \left( \frac{D\mathbf{u}_f}{Dt} - \frac{d\mathbf{u}_p}{dt}\right)\mathcal{V}_p`

.. |VirtualMass_Eq| replace:: :math:`\mathbf{F}_p,vm = -C_{p,vm} \rho_f \left( \frac{D\mathbf{u}_f}{Dt} - \frac{d\mathbf{u}_p}{dt}\right)\mathcal{V}_p`

.. |VM_Zuber_Eq| replace:: :math:`C_p,vm = (\frac{1}{2})\frac{(1+2\varepsilon_p)}{(1-\varepsilon_p)}`

.. |VM_Zuber_Eq| replace:: :math:`C_{p,vm} = (\frac{1}{2})\frac{(1+2\varepsilon_p)}{(1-\varepsilon_p)}`

.. |VM_Nijssen_Eq| replace:: :math:`\begin{split}C_p,vm =& C_0,vm \\ &+ \sum_{k=1}^3 \left\left[a_{k,0} + a_{k,1} \ln\left(\frac{\rho_p}{\rho_f}\right)\right] \varepsilon_p^k \right\end{split}`

.. |VM_Nijssen_Eq| replace:: :math:`\begin{split}C_{p,vm} &= C_{0,vm} \\ &+ \sum_{k=1}^3 \left\{ \left[  a_{k,0} + a_{k,1} \ln \left( \frac{\rho_p}{\rho_f} \right) \right] \varepsilon_p^k \right\}\end{split}`

+--------------------------+-----------------------------------------------------------------------------------------+-------------+-----------+

|                          | Description                                                                             |   Type      | Default   |