Change MAT_SPD to mean Hermitian positive definite

For real-valued applications, this merge request does not change anything.

For complex-valued applications, it changes the semantics of MAT_SPD: it now implies MAT_HERMITIAN.

If a matrix satisfies v^* M v \in \mathbb{R}_{> 0} for every v \in \mathbb{C}^n, then it is Hermitian. This means that this change cannot break any user's mathematically well-formulated code: if they thought they had a matrix that was MAT_SPD and not MAT_HERMITIAN, they had an error.