Refactor cell diameter computation to utilities

[AmishgaAlphonius]

Description

The cell diameter calculation was repeated in every scratch data. This PR refactors it in an inline function in the utilities.

Testing

No impact on current tests.

Checklist (will be removed when merged)

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[blaisb]

One small thing to change

[blaisb]

Suggested change

	double h;
	if constexpr (dim == 2)
	h = std::sqrt(4. * cell_measure / numbers::PI) / fe_degree;
	else if constexpr (dim == 3)
	h = std::pow(6. * cell_measure / numbers::PI, 1. / 3.) / fe_degree;
	return h;
	if constexpr (dim == 2)
	return std::sqrt(4. * cell_measure / numbers::PI) / fe_degree;
	else if constexpr (dim == 3)
	return std::cbrt(6. * cell_measure / numbers::PI, 1. / 3.) / fe_degree;
	else
	Assert("....")

The code does not support 1D anyway right now so might as well assert if the wrong number is called instead of returning something without an initial value.