Investment variable for node injection

[jkiviluo]

From a discussion in Amsterdam: It would be possible to reduce two variables from a situation where one want to limit the total injection from multiple sources into a node. At the moment one needs a connection and an extra node, but if investment variable could be applied directly to v_node_injection, this could be avoided.

[clizbe]

@datejada Do you know if this is worth the squeeze (yay reducing variables!) and something we're planning on doing? What priority should it have?