Add 1/s entr/detr dim scales with total entr rate option

[costachris]

This PR:

• Adds option to use [1/s] entrainment rate, rather than fractional rate [1/m]
  Fractional Rate -> E = $\rho a w \epsilon$ = $\rho a w (\gamma_m F)$;where $\epsilon$ is the fractional rate, $\gamma_m$ is a dimensional scale [1/m], F is a nondimensional function
  Total Rate -> E = $\rho a \gamma_s F$; where $\gamma_s$ is a dimensional scale [1/s].
• More consistently applies min area clipping
• Computes massflux gradient before use in entr/detr functions.