Add from_grid() factory method to the MultivariatePolynomialSingleABC.

A factory method from_grid() has been added to the abstract class MultivariatePolynomialSingleABC and is, therefore, available to all concrete polynomial classes. This method allows the creation of polynomial instances with a Grid instance; the multi-index set is taken from the given grid.

The test suite has been updated to accommodate these changes.

This commit should resolve Issue #160 (closed).