Allow exptime to be entered as a string fraction (#1367)

[wtgee]

Closes #1367. This PR allows the camera times and scheduler observation times to be entered as a string fraction, such as '1/4'. Additionally, it updates the Canon DSLR driver to ensure that if the discrete shutterspeed table clips the requested exposure time (e.g. for tiny values like '1e-20'), the actual clipped physical shutterspeed is written to the FITS header instead of the requested value.

[codecov]

Codecov Report

❌ Patch coverage is 93.49593% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.61%. Comparing base (834b5af) to head (905aa2f).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_camera.py	90.76%	4 Missing and 2 partials ⚠️
src/panoptes/pocs/camera/simulator/dslr.py	94.73%	0 Missing and 1 partial ⚠️
src/panoptes/pocs/scheduler/observation/base.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1461      +/-   ##
==========================================
+ Coverage   69.29%   69.61%   +0.32%     
==========================================
  Files         102      102              
  Lines        9870     9990     +120     
  Branches      846      856      +10     
==========================================
+ Hits         6839     6955     +116     
- Misses       2852     2853       +1     
- Partials      179      182       +3     

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