Delay parsing trial times

[ageorgou]

Addressing a TODO comment from earlier version. At the moment we only have tab-separated lines in the Intertrial FIFO Times section (the trigger times indicating beginning and end of trials) so we may not need the extra generalisation this gives. Even so, this moves the parsing of those lines to the relevant place and keeps the original text (without any casting) in the raw fields.

• [x] Move parsing and casting of start/stop times away from generic file reading
• [x] Test on new-format data!
• ~(possibly) Cater for cases when stop-time of the last trial is missing~ done in #35

[ageorgou]

Will not merge until #35 is done, so we have some tests.

[codecov-io]

Codecov Report

Merging #34 into new-labview will decrease coverage by 1.03%. The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           new-labview      #34      +/-   ##
===============================================
- Coverage        54.88%   53.84%   -1.04%     
===============================================
  Files               14       15       +1     
  Lines             1629     1690      +61     
  Branches           254      265      +11     
===============================================
+ Hits               894      910      +16     
- Misses             698      743      +45     
  Partials            37       37              

Impacted Files	Coverage Δ
src/silverlabnwb/header.py	87.12% <100.00%> (+1.40%)	:arrow_up:
src/silverlabnwb/metadata_gui.py	0.00% <0.00%> (ø)
src/silverlabnwb/nwb_util.py	0.00% <0.00%> (ø)
src/silverlabnwb/metadata.py	88.63% <0.00%> (+0.26%)	:arrow_up:
src/silverlabnwb/signature.py	57.89% <0.00%> (+0.34%)	:arrow_up:

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