Implement weighting for individual IODs for both orbit-fit and rms-reporting

[interplanetarychris]

Create mechanism to provided weighted consideration of individual IODs, with concepts ranging from:

1. all "100%" (all equal)
2. heavier weight to most-recent observations
3. heavier weight to higher-ranked observers
4. Low/zero weight to outlier observations from low-ranked observers
5. Equal weight to observation sessions with min number of observations (e.g. 100 observations on one pass have the same weight as 3 observations on another pass)

First implementation of this would provide for manual specification of weights from UI in satfit.py