[Bridges] Maintenance of GeoMeanBridge

[odow]

I was trying to address this TODO:

MathOptInterface.jl/src/Bridges/Constraint/geomean.jl

Lines 48 to 52 in 5fb36a4

	# Initially, (t, x) is of dimension d so x is dimension (d-1)
	# TODO the sentence below is a little confusing, the n isn't used anywhere
	# We create n new variables so that there are 2^l = d-1+n variables x_i
	# We then need to create 2^l-1 new variables (1+2+...+2^{l-1})
	# So the total number of variables is d+2^l-1

and I got very confused with the GeoMean implementation.

I started with some renaming, but then I gave up. It could use some much better documentation.

[odow]

@blegat do you have the pdf of the book? (Or want to expand the documentation?)

[odow]

Merging this for now. I'll come back with more documentation.