[ENH] index sessions.tsv and scans.tsv

[Remi-Gau]

• index scans.tsv and sessions.tsv as strings to the full path for those files
• add rudimentary tests to make sure they are picked up

fixes #145
fixes #164: This does not interfere with querying for modality as scans.tsv and sessions.tsv are not stored within structures

Note / Explanation:

• only the filename is stored to stick to the plan to minimize how much "data" is loaded at indexing.
• querying for the content and metadata of those files is not implemented yet.

[codecov]

Codecov Report

Merging #182 (dc8ca29) into dev (c987040) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff          @@
##             dev    #182   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         25      25           
  Lines        930     931    +1     
=====================================
- Misses       930     931    +1     

Flag	Coverage Δ
unittests	0.00% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
+bids/layout.m	0.00% <0.00%> (ø)

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