Infer upkeep/production from buying/selling at different ticks

[valitas]

If

• we just sampled the current buying/selling figures of a building, and
• the building has no known upkeep/production figures (maybe because it has only ever been seen from the sector index, or is a MO), and
• we have also the buying/selling figures of that building sampled at a previous tick,

then we should try to guess the upkeep/production figures from the number of ticks elapsed and the difference between those samples.

This would be nice because it is a way to make projectable all those buildings we can track from the sector index. More importantly: it's the only way to finally close #32. This really is the only way we're ever going to get the upkeep figures of a MO, short of having the user provide the grid setting manually, with #38 again.

Do note: the figures we get from this are unreliable: someone may have traded with the building in between samples, throwing off the figures. So, when we do this, we need to check if the numbers seem sane (e.g. all buying/selling figures increased by the same ratio, or some such) before using them.