Expose battery state of charge and camera record time

[aksell]

Description

1. Add a property for reading the battery relative state of charge and the camera record time.
2. Document the camera class with return values and the valid ranges for the different camera parameters.

Also add a test for the depth property because it was not tested, and the coverage bot always wants more.

Checklist before merging

• [x] Run the tests while connected to a drone
• [ ] Update the package version according to semver

[codecov[bot]]

Codecov Report

Merging #63 into master will increase coverage by 0.64%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage    67.1%   67.75%   +0.64%     
==========================================
  Files           6        6              
  Lines         301      307       +6     
==========================================
+ Hits          202      208       +6     
  Misses         99       99

Flag	Coverage Δ
#macos_python37	67.75% <83.33%> (-32.25%)	:arrow_down:
#macos_python38	67.75% <83.33%> (-32.25%)	:arrow_down:
#ubuntu_python37	67.75% <83.33%> (+0.64%)	:arrow_up:
#ubuntu_python38	67.75% <83.33%> (+0.64%)	:arrow_up:
#windows_python37	100% <ø> (+32.89%)	:arrow_up:
#windows_python38	100% <ø> (+32.89%)	:arrow_up:

Impacted Files	Coverage Δ
blueye/sdk/pioneer.py	70.29% <100%> (+1.92%)	:arrow_up:
blueye/sdk/camera.py	51.51% <66.66%> (+0.72%)	:arrow_up:

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