Add emission_weight (AcademySoftwareFoundation#231)

Implements the change described in AcademySoftwareFoundation#229.
# Conflicts:
#	index.html
#	parametrization.md.html

Author: portsmouth

index.html

@@ -1623,9 +1623,6 @@
 
 [^ingamut]: Note that we assume that all RGB colors are in-gamut, and that colors representing albedos have components in the $[0,1]$ range (otherwise RGB renderers do not produce sensible results). The `emission_color` however is the exception which is permitted to have arbitrarily large (positive) components, since it represents an arbitrary luminance multiplier.
 
-i.e. have components in the $[0,1]$ range in the
-specified color space.
-
 [^Jacobian]:  Omitting Jacobian factors.
 
 [^normalization]:  Omitting normalization factors.