chern -- compute Chern classes of a sheaf

Ways to use chern:

• chern(AbstractSheaf) -- compute the total Chern class of a sheaf
• chern(ZZ,AbstractSheaf) -- compute a Chern class of a sheaf
• chern(ZZ,ChernClassVariableTable) -- get value of a Chern class variable
• chern(ZZ,Symbol) -- make a Chern class symbol
• chern(ZZ,ZZ,AbstractSheaf) -- compute several Chern classes of an abstract sheaf

For the programmer

The object chern is a method function.