Make usages of Eigen::array compatible with std::array.
Make usages of Eigen::array compatible with std::array.

Eigen::array is no longer necessary, so will be deprecated/removed and replaced
with `std::array`.  The main difference is the constructor - currently Eigen::array
allows `array(a, b, c, ...)` construction, whereas `std::array` requires an initializer
list.  We also need to remove any direct access to the `Eigen::array::values`
internal parameter, in favor of regular index access.
