Fix cache for dask.dataframe

[hoxbro]

When running the bikes example, I got the following errors:

2022-08-24 10:12:20,755 Could not cache 'occupancy' to parquet file. Error during saving process: [Errno 21] Failed to open local file '/home/shh/Development/holoviz/repos/lumen/examples/bikes/cache/occupancy.parq'. Detail: [errno 21] Is a directory

This PR does three things:

1. Trying to debug this, I found it confusing with the combination of os.path and pathlib.Path, so for sources/base.py, I converted it all to Path. I also set root up as a param, to always have it as Path.

2. The extension of the bikes URL https://api.tfl.gov.uk/Occupancy/BikePoints/@{stations.stations.id}?app_key=a1c692de000b4944af55f59d8e849915 was before this PR id}?app_key=a1c692de000b4944af55f59d8e849915 and with this PR None

3. dd.to_parquet in _set_cache first created a directory named occupancy.parq with part.0.parquet files. This means that the final result file occupancy.parq could not be saved as it was already a directory. This meant that the cache failed, gave the error message above and removed all the cache, which is not desired. I do it now by removing the extension if data is a dask.DataFrame. When loading in cache in _get_cache I check for directories with and without the extension

[codecov-commenter]

Codecov Report

Merging #305 (c5ab64f) into master (fdfdc6e) will increase coverage by 0.26%.
The diff coverage is 74.35%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
+ Coverage   66.04%   66.31%   +0.26%     
==========================================
  Files          59       59              
  Lines        6147     6189      +42     
==========================================
+ Hits         4060     4104      +44     
+ Misses       2087     2085       -2     

Impacted Files	Coverage Δ
lumen/sources/base.py	62.65% <72.22%> (+0.92%)	⬆️
lumen/tests/sources/test_base.py	99.05% <100.00%> (+0.18%)	⬆️
lumen/tests/sources/test_derived.py	100.00% <0.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.