Add selectors and boundary flow source diagnostics

[mikekryjak]

2D particle and energy balances are challenging. One of the ways this is unwieldy is that the boundary flows must be calculated from flow diagnostics and care must be kept to select the right cells (e.g. the perpendicular flows are on the xlow side, which means you need to select the final domain cell at the PFR, but the guard cell at the SOL).

This PFR adds new diagnostics where the boundary flows are represented as sources in the final domain cells, making it a lot easier to tally up where things are going.

It also adds a few new selectors, adds the toroidal cell length variable and renames dl to dpol.