Add an option to have a default count of 1 for all terms when calculating IC

In theory the MRCA should always have an IC if the IC table is kept in sync with the database, but if the database is out of sync we can imagine scenarios where the MRCA is to a new term with no IC calculated. Rather than treat as IC = infinity, instead treat each term has having a minimum count of one (semi-open world assumptions - assume if a term exists it is instantiated somewhere)

Locate the information-content calculation and its option or configuration path. Check how a missing IC for an MRCA is currently handled, then use the relevant existing IC tests to verify an opt-in default count of one; done means the new option is available and missing terms no longer produce infinite IC when it is enabled.

Written by the indexing model from the issue text.