Initial commit: interface for a class to manage the storage of data for
ContactManager. As can be seen from this, I have decided to follow the strategy
of storing our meetings internally as the specific subclasses Past &
FutureMeeting, i.e. within the classes we store as much information as
possible. Only in certain situations will it be necessary to "upcast to
forget". (This will require changes to ContactManagerImpl)
update -- forgot to include methods for reading/ writing files!
added tests for ContactImpl;
alternative constructor for ContactImpl without notes param.