Ppu history - Githubissues

Proper fixes and terminology to get us ready for historical position lifetimes as described in #345.

More to come in terms of explanation... or not 😂

[x] could we maybe just drop the top level .size and be_price values ?
- so we can't (yet), because the ems uses the Position type internally as well and doesn't track all the clears (since that would be more msging overhead) so I think maybe having these is fine but we should probably change .be_price -> .ppu?
- and instead just arrange the clears table to make these rolling values more obvious?, for eg:
```
[kraken.spot."xbteur.kraken"]
bsuid = "xbteur"
clears = [
{  accum_size = 0.00639943, ppu = 19620.4996648139, cost = 0.32477, price = 19519.0, size = 0.00639943, dt = "2022-06-17T00:17:53.030664+00:00", tid = "TZVEXD-USARI-COY2ZR"},
```
  which would be easiest to read if we sort clears reverse chronological so that the most recent trades (and thus their ppu and accum_size entries would be closest to the top?
[x] it follows that maybe we should adjust the column order for each clears entry maybe to: dt, ppu, accum_size, price, size, cost, tid?
~[ ] i was hoping to have an indent per account (mentioned it in #345 as part of a custom encoder) such that it's distinguishable which pps are grouped together by backend, my only concern is then the clears table entries will be a bit cramped horizontally?~ delaying this one again

pikers / piker