make lowrankupdate/downdate more permissive

[bicycle1885]

This makes lowrankupdate and lowrankdowndate function arguments more permissive. The following code didn't work because StaticArray was not a subtype of DenseArray.

using StaticArrays
using LinearAlgebra
A = @MMatrix [1.0 0.0; 0.0 1.0]
C = cholesky(A)
lowrankupdate(C, @MVector [1.0, 2.0])

[stevengj]

Can we have a test? e.g. use a range

[bicycle1885]

Thank you for your comment. I've added test cases for a new abstract vector type (ranges are not usable in this case because they are immutable) and slightly cleaned up the test code.