BB-15: Query components of monoids and semirings

[mcmillan03]

GxB_Monoid_operator() return the monoid operator
GxB_Monoid_identity() return the monoid identity value

GxB_Semiring_add() return the additive monoid of a semiring
GxB_Semiring_multiply() return the binary operator of a semiring

[tgmattso]

I want introspection of all graphBLAS objects and Types. So yes, I want this.

[manoj63]

To be consistent, you may also want the identity elements of the additive and multiplicative monoids of the semi-ring.

It may be useful to work out use cases where this capability is used to eliminate chances of errors under program edits, or creating of more concise code.