Create net IOB line in NS to visualize insulin

[danamlewis]

With the advent of SMB, it is common to get plenty of insulin, yet it be visualized as "zero" temps and SMB's, causing lack of understanding about how much insulin is active. This will be even more common with future oref0 versions where more of the delivery can be done with SMB.

Proposed solution: create a new IOB line that will replace the 'render basal' blue line in Nightscout. It can be visualized similarly with 0 netIOB as the dotted line, and show positive or negative IOB relative to the line.