Extract pressure sensor out of TethysCommPlugin

[mabelzhang]

As verbally discussed, ocean pressure, added in #91 to TethysCommPlugin in the interest of time, should be extracted into its own custom sensor, along with other science sensors in ScienceSensorsSystem.cc, for upstreaming to ign-gazebo.

Note that until longitude/latitude are implemented, the output is meaningless, because pressure is computed from latitude and depth.

Science sensor data can then be packaged (by us or end users) into a CTD (conductivity, temperature, depth) sensor, which requires salinity (gives conductivity), temperature, and depth (calculated from pressure, but can be ground truth from simulation).

[mabelzhang]

Do as part of #100 while upstreaming