some sum optimisations

[tias]

to implement, e.g.
e = sum(x[:2]) == sum(x[2:])
flatten_constraint(e)

[tias]

in a new 'simplify' transformation?

[IgnaceBleukx]

Another possible optimization:
Current situation:

e = wsum([3,5], [x,y])
1 + e = sum(1, e)

Improvement:
1 + e = wsum([1,3,5],[1,x,y])

Hmm, although const + 3 * x + 5 * y is the standard form of a LinExpr...
Not sure we should convert it then...

[Wout4]

@tias what should this get simplified to?