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

[cmungall]

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)