Align interfaces for discrete and continuous calculators

[jlizier]

This will involve:
-- having the continuous calculators auto-calling startAddObservations() (at first use of addObservations if not already called) and finaliseAddObservations() (at first calculation). This will certainly make it easier for users who sometimes stumble over this.
-- adding setProperties() and getProperties() to the discrete calculators. This will be required e.g. for implementing auto-embedding for TE anyway.