Removing the concept of Continuity Index.

This means that continuities will be indexed by their symbol name, which is more natural and what most people have been using.
This change means that some older files will not be able to be opened by calchart 3.4 and beyond.  I've created an error that will pop up when that happens.
Tested by opening older files and running their continuities.
Tested by opening and saving a file in the new format, then opening again.