require size field to be linear, not equal-order
require size field to be linear, not equal-order

this assert required size fields to be the
same order as the coordinate field, but
for various reasons we usually keep size
fields linear.
one of those reasons (the mid-edge placement
math) has recently been removed, but other
things like the SPR error estimator or
the size field transfer code probably
retain linear assumptions.

YetAnotherMinion/core,YetAnotherMinion/core,YetAnotherMinion/core,yangf4/core,YetAnotherMinion/core,yangf4/core,YetAnotherMinion/core,yangf4/core,yangf4/core,yangf4/core