Dense -- an optional argument

Functions with optional argument named Dense:

• mutableIdentity(...,Dense=>...) -- see mutableIdentity(Ring,ZZ) -- make a mutable identity matrix
• mutableMatrix(...,Dense=>...) -- see mutableMatrix -- make a mutable matrix
• randomMutableMatrix(...,Dense=>...) -- see randomMutableMatrix(ZZ,ZZ,RR,ZZ) -- a random mutable matrix of integers