A field containing one or more plane equations.
SoMFPlanes are written to file as groups of four floating point values separated by whitespace. In each set of four values, the first three are the normal direction
of the plane, the fourth is the distance of the plane from the origin (in the direction of the normal).
When more than one value is present, all of the values are enclosed in square brackets and separated
by commas; for example:
[ 1 0 0 0, .707 .707 0 100, ]