Positivity check in PDiagMat and ScalMat constructors?

[dmbates]

At present there is no check for positivity of the arguments that will be on the diagonal of a PDiagMat or ScalMat. Probably a good idea to check that.

julia> using PDMats

julia> ScalMat(4,-1.5)
ScalMat(4,-1.5,-0.6666666666666666)

julia> PDiagMat([-1.,2.,-3.,4.])
PDiagMat(4,[-1.0,2.0,-3.0,4.0],[-1.0,0.5,-0.333333,0.25])

[andreasnoack]

I think we need this. In particular now that we've also defined isposdef in #123.