Update single droplet evaporation documentation

[d-montgomery]

This PR has two updates:

1. Renamed the posf_nonreacting mechanism and added heptane, decane, and iso-octane
2. Documentation for the spray model. Specifically, the single droplet evaporation validation cases which were originally run with PeleMP here. The updated code that performs the validation tests and generates the figures is included in a new SprayEvap RegTest in PeleLMeX (PR #521).

The SprayEvap case in LMeX includes two new validation cases:

• Nomura et al. (heptane droplet)
• Wong & Lin (decane droplet in cross flow)

Note: I did not remove the original plots since they were used in the original PeleMP paper (doi: 10.1115/1.4064494)

[baperry2]

Need to add the mechanism name change to the list_mech file.

Test results look good, thanks for adding these!

[d-montgomery]

Need to add the mechanism name change to the list_mech file.

Test results look good, thanks for adding these

I always forget the list_mech file. Thanks for catching that. I made the other changes you suggested as well.

[baperry2]

We should really automate the list_mech thing at some point

[d-montgomery]

These last few commits include a bug fix for 2D spray that was brought up in the LMeX discussions. Previously, the cell volume in two-dimensional simulations for the gas-phase source terms from spray were defined as $V_{cell} = \Delta x \Delta y$. This lead to discrepancies in the gas-phase mass fraction, temperature, and velocity.

The spray model assumes spherically symmetric droplets and is inherently three-dimensional. For two-dimensional simulations, the domain is now treated as one cell wide in the z-direction, with $L_z = \Delta x$, so the volume in the gas-phase source terms is: $V_{cell} = \Delta x \Delta y \Delta x$.

For single droplet cases, this effectively places an infinite array of droplets spaced $\Delta x$ apart in $z$, exaggerating Stefan flow in the x- and y-directions and omitting flow in z. While this has minimal impact on droplet diameter or temperature, it can distort the surrounding gas-phase flow, especially for low $\text{Re}_d$.