Added EMDA metric

[fjebaker]

Added the Einstein-Maxwell Dilatron Axion (EMDA) metric, from Younsi et al. (2021), which is a non-GR metric examined as part of the EHT 2021 Sgr A* paper series.

Example: shadow of the EMDA metric

using Gradus
using StaticArrays

using Gradus, StaticArrays
m = DilatonAxionAD(M=1.0, a=0.8, b=0.7, β=0.2)

u = @SVector [0.0, 1000.0, deg2rad(90), 0.0]

img = @time rendergeodesics(
    m, u, 2000.0; 
    abstol=1e-9, reltol=1e-9,
    image_width=256, 
    image_height=256, 
    fov_factor=13.0,
    verbose = false,
    # since inner_horizon is ill-defined
    dtmin = 1e-8
)

Plotting:

[fjebaker]

There's probably some typos in the mathematics here, but it seems to trace okay.