Find the coefficients of the plane equation

## Arguments

- p
A point on the plane (or N x 3 matrix of multiple points)

- n
vector normal to the plane (or N x 3 matrix of multiple vectors)

## Value

a matrix with 4 columns `a,b,c,d`

where ```
ax + by + cz + d =
0
```

## Details

Both `p`

and `n`

can accept multiple points/vectors to
calculate many planes at once.

## Examples

```
# Mushroom Body Entry Point - plane perpendicular to axon tract as
# projection neurons enter mushroom body calyx
mbe=plane_coefficients(p=c(207, 102, 142), n=c(.6,-0.1,0.3))
if (FALSE) {
plot3d(Cell07PNs)
planes3d(mbe[1:3], d=mbe[4])
}
```