MAINT: remove fastparquet compression for bubblesam parquet

[adamwitmer]

In line with #4 (comment), this PR simplifies bubblesam parquet saving/loading by removing the use of fastparquet engine, which saved objects as strings. This change will also simplify parquet loading in PR #5 and remove the need for the ast module there.

[adamwitmer]

I also opened issue #40 to make a note of the need to simplify data structure handling for saving as parquet, which I did not address in this PR to keep the scope limited to changes related to fastparquet.

[tylerjereddy]

Can you provide a brief/clear explanation of why you need to remove fastparquet usage if it only stores objects as strings and you're going to remove the object storage anyway? For example, a reviewer might sensibly deduce that if objects are removed from the control flow then the problem goes away without needing to mess around with the parquet format in the first place.