Improving external feature interface and documentation

[smauq]

• Added new sensor to EuRoC config
• Removed maplab_features as a default dependency as it will unnecessarily pull in unwanted packages if not desired. Instead will add a link in the ReadMe and a wiki page detailing the separate installation here Updating the wiki #375
• Updated some interfaces in aslam
• Fixed a few return value conversions from aslam functions with descriptor types that were causing unnecessary copies to happen
• Added an outlier rejection filter to the matches provided by external keypoint tracking

[ethzasl-jenkins]

Test PASSed.

[smauq]

Multiple failed Jenkins builds are due to Github's RSA key changing.