support non-zero EB/TB covariance

[kimmywu]

CMBLensing currently doesn't support non-zero EB/TB covariance in its signal/noise covariance construction. See

CMBLensing.jl/src/flat_s0s2.jl

Lines 73 to 84 in 292bcea

	# A Flat TEB covariance of the form:
	#
	# [ΣTT ΣTE ⋅
	# ΣTE ΣEE ⋅
	# ⋅ ⋅ ΣBB]
	#
	# We store the 2x2 block as a 2x2 SMatrix, ΣTE, so that we can easily call sqrt/inv on
	# it, and the ΣBB block separately as ΣB.
	struct FlatIEBCov{T,F} <: ImplicitOp{IEBFourier,S02,Pix}
	ΣTE :: SMatrix{2,2,Diagonal{T,F},4}
	ΣB :: Diagonal{T,F}
	end

Would be good to add that feature to allow modeling of correlated EB/TB noise/signal. I haven't thought through all the details of how to implement this yet. So this is a reminder.