Consistently use template parameters in VmathArray

Issue/feature addressed

The Vmul and Vvtvp functions in VmathArray assume NekDouble variable type for the 2D array argument instead of using the template parameter T. This MR fixes this issue.

Proposed solution

Implementation

Tests

Suggested reviewers

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Notes

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Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).