Unify CTD and CTD_BGC to one instrument

[j-atkins]

Now that we have implemented the sensors refactoring in #328, the CTD and CTD_BGC instruments can nicely be consolidated to one CTD instrument which supports both physical and biogeochemical variable sampling (configurable via the expedition YAML).

This PR also updates the documentation to reflect these changes and updates/adds relevant tests.

Closes #260

[erikvansebille]

Nice cleanup! But I now understand that a CTD can only be run with BGC variables; correct? So no option to use only a subset of the sensors? Would that be useful and/or authentic?

[erikvansebille]

Is there still a possibility to use only the physical variables? Or will a ctd now always also use bcc?

[j-atkins]

Nice cleanup! But I now understand that a CTD can only be run with BGC variables; correct? So no option to use only a subset of the sensors? Would that be useful and/or authentic?

Ah yes it is indeed possible to only use the physical variable sensors still (or any subset/combination of phys/bgc variables)! Which sensors are on/off is configurable in the instrument config section of the expedition YAML.

The docs and virtualship plan tool still need updating to reflect the sensor logic changes. I've made a couple of issues just now (#335, #334) to track these.

Related and maybe useful to know: this doesn't mean user can add a 'non-sensical' sensor to an instrument. E.g. you can't add a BGC variable to an ADCP. There are still controls on which sensors an instrument can take.