fix stimulus, marks and trials

[jihyunbak]

Description and related issues

List of changes:

• use TDT-loaded mark onsets when available, instead of using mark thresholds (as discussed #102); this would resolve #76
• create a single trial for continuous stimuli (per discussion in issue #88)
  • add new class SingleTokenizer for this
• update name and description for mark and wav TimeSeries objects (related issue: #51)
  • (still open for discussion)

This PR would close

• Close #51
• Close #76
• Close #88
• Close #102

Checklist:

• [ ] All tests pass on catscan: run pytest --basetemp=tmp -sv -n 8 tests on catscan from the root directory
  • test_catscan passes on all test blocks with resample_data=False
  • larger blocks fail with resample_data=True (RVG16_B06, RVG16_B07, RVG16_B10, RVG21_B13, R56_B10)
• [x] If needed, docs have been update: docs/source has been updated for any added, moved, or removed files
• [x] Docs build with no errors: run make clean & make html from the docs folder
• [x] No python formatting errors: run flake8 nsds_lab_to_nwb tests from the root directory

[jihyunbak]

All test blocks in test_catscan passes without resampling, while some larger blocks fail with resampling (this has been the case recently). For the sake of fixing the state of the repo before tomorrow's dev meeting, I will go ahead and merge this PR as well ...

[jihyunbak]

Let's talk about future PR rules