Clean up Polynomial Coefficients as a Property of MultivariatePolynomialSingleABC

Polynomial coefficients are stored in the property coeffs which also includes a setter that will verify any given values before they are assigned. The coefficients of a polynomial themselves may first be None, in which case the polynomial is termed "uninitialized" in the codebase. Accessing the coefficients of an uninitialized polynomial raises an exception which I find rather unintuitive. It does, however, provide a single error point for attempting operations that require initialized polynomials (__add__, __mul__, __neg__, etc.) on uninitialized polynomials as long as the (public) property is accessed (i.e., coeffs instead of _coeffs).

Refactor and clean up the docstrings of coeffs property; in particular perhaps introduce a better encapsulation of the verification procedure for polynomial coefficients. For instance, while currently the coefficients may be assigned integer values, most of the underlying Numba JIT-compiled implementations require specifically numpy.float64 type for the coefficients and will raise TypeError if another type is supplied (most prominently, perhaps, int types). There might be a case to "normalize" the stored coefficient values to strictly numpy.float64 type because that's the type that the Numba functions can operate with anyway.