Add gphoto utils

[wtgee]

Two class methods added to the gphoto base camera class: start_tether and gphoto_file_download. These are meant as utility helpers and will likely get a cli wrapper in the future if useful.

Comes from #1167

[codecov[bot]]

Codecov Report

Merging #1172 (531e76c) into develop (4908119) will decrease coverage by 8.64%. The diff coverage is 56.26%.

@@             Coverage Diff             @@
##           develop    #1172      +/-   ##
===========================================
- Coverage    83.08%   74.44%   -8.65%     
===========================================
  Files           86       91       +5     
  Lines         7425     7782     +357     
  Branches       635      864     +229     
===========================================
- Hits          6169     5793     -376     
- Misses        1083     1849     +766     
+ Partials       173      140      -33     

Impacted Files	Coverage Δ
src/panoptes/pocs/camera/gphoto/remote.py	0.00% <0.00%> (-21.57%)	:arrow_down:
...rc/panoptes/pocs/scheduler/observation/compound.py	0.00% <0.00%> (ø)
src/panoptes/pocs/sensor/power.py	40.23% <0.00%> (-1.74%)	:arrow_down:
...rc/panoptes/pocs/state/states/default/analyzing.py	5.55% <0.00%> (-54.45%)	:arrow_down:
src/panoptes/pocs/state/states/default/tracking.py	6.66% <0.00%> (-76.67%)	:arrow_down:
src/panoptes/pocs/utils/cli/main.py	0.00% <0.00%> (-70.00%)	:arrow_down:
src/panoptes/pocs/utils/database.py	0.00% <0.00%> (ø)
src/panoptes/pocs/utils/error.py	0.00% <0.00%> (ø)
src/panoptes/pocs/utils/logger.py	100.00% <ø> (ø)
tests/test_ioptron.py	66.66% <0.00%> (+0.70%)	:arrow_up:
... and 70 more

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