Support FREQ_CENTROID and EFFECTIVE_BW per sample in BDA code

* Codex Africanus version: 0.3.1
* Python version: 3.8
* Operating System: Ubuntu 20.04

### Description

Similarly to the way `TIME_CENTROID` and `EXPOSURE` are the `effective` versions of the `nominal` `TIME` and `INTERVAL` columns, we need `FREQ_CENTROID` and `EFFECTIVE_BW` as effective versions of `CHAN_FREQ` and `CHAN_WIDTH`, respectively.

/cc @o-smirnov @landmanbester 