In N(), add support for multicolumn offsets.

[krivit]

This references this TODO:

ergm.multi/R/InitErgmTerm.multinet.R

Lines 327 to 334 in 4c6762b

	## the same number of stats. lm-offset does not work with this
	## approach at this time, either *unless* all terms are offset by
	## the same amount (that may vary over networks). Also,
	## N.compact_stats can be set to FALSE to force the general case to
	## be used.
	# TODO: A more refined detection than is.curved(), since currently
	# all operators show up as curved.
	# TODO: Check that multivariate offset is still not allowed in R.

Multivariate offsets have been supported since R 3.6 or so.