Use of `_get_array()` instead of `get_array()` causes failures when getting element results

[rileychall]

The PostProcessing.element_values() method uses _get_array() where it appears that it should be using get_array(), making it vulnerable to timeouts.
https://github.com/pyansys/pymapdl/blob/c7c438d81c4a01ca9ba6fa33767a719278013107/src/ansys/mapdl/core/post.py#L421