Refactor `RefElemData`

[jlchan]

Currently dispatch on approximation_type is messy and makes it hard to add structure based on quadrature structure (e.g., Stroud tensor product quadrature).

This PR reorganizes RefElemData so that quadrature information goes into the Polynomial approximation type. This is consistent with the current approach taken by SBP approximation type, which also encodes specific information about the quadrature used to construct operators.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 96.84%. Comparing base (4fd36c7) to head (1501989).

:exclamation: Current head 1501989 differs from pull request most recent head 1e5796f. Consider uploading reports for the commit 1e5796f to get more accurate results

Files	Patch %	Lines
src/RefElemData.jl	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #157 +/- ## ========================================== + Coverage 96.78% 96.84% +0.06% ========================================== Files 25 25 Lines 3265 3268 +3 ========================================== + Hits 3160 3165 +5 + Misses 105 103 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[jlchan]

To clean up the Polynomial approximation type, we'll take the following steps:

• we now default to TensorProductQuadrature on Quad and Hex elements
• we introduced MultidimensionalQuadrature instead of DefaultPolynomialType to distinguish from TensorProductQuadrature.
• DefaultPolynomialType is now only be used for dispatch to select between MultidimensionalQuadrature and TensorProductQuadrature.