Add bpz-demo-data option to get-data command

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c768b4a) 97.12% compared to head (51aab75) 97.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #55 +/- ## ======================================= Coverage 97.12% 97.13% ======================================= Files 32 32 Lines 1671 1673 +2 ======================================= + Hits 1623 1625 +2 Misses 48 48 ``` | [Files](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [src/rail/cli/options.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvY2xpL29wdGlvbnMucHk=) | `90.69% <100.00%> (+0.22%)` | :arrow_up: | | [src/rail/cli/scripts.py](https://app.codecov.io/gh/LSSTDESC/rail_base/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3JhaWwvY2xpL3NjcmlwdHMucHk=) | `89.42% <100.00%> (+0.10%)` | :arrow_up: |

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[OliviaLynn]

My general plan is that all standard data would go into "data_files." The bpz data is only quarantined as a separate flag because it's dummy data that should very much not be used for anything else.

(While I mention it in the help description of the flag, I realize it would probably be nice to add a comment in the get_data function itself. Just added a new commit.)

Please feel free to let me know if you have a different plan in mind.

[eacharles]

Ok, makes sense. Thanks