Tidy up file structure and naming conventions

[thompson318]

This addresses some of the file naming issues identified in #103.

Replaces underscore with . see : #103 (comment)

Places the ehr files in the same directory as the waveform data pseudonymised\{date}\{stem}.ehr.csv to address #103 (comment).

I haven't been able to to setup get_ehr_lookup as the single source of truth yet and am unlikely to be able to get to have time this week.

The file names will need changing to pseudonymised\{date}\{stem}.ehr.parquet in a separate change.

Feel free to merge this if and when convenient.

The linting error occurs on code which the same as the target branch where linting is currently skipped, so I've left it alone for now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.68%. Comparing base (f6944e1) to head (56a1ac6).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           ehr_queries_clean     #105      +/-   ##
=====================================================
- Coverage              74.71%   74.68%   -0.03%     
=====================================================
  Files                     18       18              
  Lines                    870      869       -1     
  Branches                  79       79              
=====================================================
- Hits                     650      649       -1     
  Misses                   193      193              
  Partials                  27       27              

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