Allow passing elements to OperationTable not closed under operation

[venczelzs]

Use the new parameter closed in OperationTable.__init__ to allow generating an operation table if the argument elements is passed and those elements are not necessarily closed under the operation.

A motivating example of usage is to quickly check if the product of two 4-cycles can be a 3-cycle:

sage: S4 = SymmetricGroup(4)
sage: a = S4([(1,2,3,4)])
sage: L = [g for g in a.conjugacy_class()]
sage: from sage.matrix.operation_table import OperationTable
sage: T = OperationTable(S4, operation=operator.mul, elements=L, names='elements', closed=False)
sage: T
         *   (1,2,3,4)  (1,2,4,3)  (1,3,2,4)  (1,3,4,2)  (1,4,2,3)  (1,4,3,2)
          +------------------------------------------------------------------
 (1,2,3,4)| (1,3)(2,4)    (1,4,2)    (1,4,3)    (2,4,3)    (1,3,2)         ()
 (1,2,4,3)|    (1,3,2) (1,4)(2,3)    (1,4,2)         ()    (1,3,4)    (2,3,4)
 (1,3,2,4)|    (1,4,2)    (2,3,4) (1,2)(3,4)    (1,4,3)         ()    (1,2,3)
 (1,3,4,2)|    (1,4,3)         ()    (1,2,3) (1,4)(2,3)    (2,4,3)    (1,2,4)
 (1,4,2,3)|    (2,4,3)    (1,3,2)         ()    (1,2,4) (1,2)(3,4)    (1,3,4)
 (1,4,3,2)|         ()    (1,3,4)    (2,3,4)    (1,2,3)    (1,2,4) (1,3)(2,4)

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[venczelzs]

Hi @kwankyu, thank you for your comments! I updated the PR, could you please take a look at it?

[kwankyu]

LGTM.