layers that are normally automatically generated to be supplemented
with additional geometry in the form of properties. The first
commit implements a CIF operator "mask-hints" that tells CIFGenLayer
that additional geometry may be specified with a property named
"MASKHINTS_" plus the name passed to the operator as its only
argument. A more extensive commit to be done will allow this
operator to be used on cifinput to use mask hints to retain the
exact geometry of mask layers used in the input file.
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