Representation of glue, skips, stretchy dimensions; extends LaTeXML::Common::Dimension.
Creates a Glue object. $spec can be a string in the form that TeX recognizes (number units optional plus and minus parts). Alternatively, the dimension, plus and minus parts can be given separately as scaled points (fixpoint), while $pfill and $mfill are 0 (when the $plus or $minus part is in scaledpoints) or 1,2,3 for fil, fill or filll, respectively.