Remove prefect dependency

[zaneselvans]

In anticipation of doing another software release, and our decision to
use Dagster as our orchestration framework, I'm removing the prefect
software package dependency, and also the DataFrameCollection module,
since Dagster has built-in dataframe persistence through pickled
dataframes on disk.

[codecov]

Codecov Report

Merging #1529 (04272b0) into dev (7d3c8b9) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             dev   #1529     +/-   ##
=======================================
- Coverage   84.2%   84.1%   -0.2%     
=======================================
  Files         66      65      -1     
  Lines       7307    7205    -102     
=======================================
- Hits        6155    6060     -95     
+ Misses      1152    1145      -7     

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