Add function for the spacetime derivative of goth(g)

[ncorsobh]

Proposed changes

Adds a function to compute the spacetime derivative of goth(g), defined as the product of the spacetime metric and square root of the absolute value of its determinant.

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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Further comments

This quantity is defined on p. 466 in Misner, Thorne, and Wheeler, Gravitation.

[wthrowe]

Also test by making up values for the metric (doesn't have to be meaningful, just some smooth functions) and taking a numerical derivative.

[ncorsobh]

After further discussion between us, we decided that this one would not be worth the effort of pursuing.

[ncorsobh]

N.B. the latest update includes a modification to the function switching the da_lapse argument with a dt_lapse and deriv_lapse, since these are calculated separately in GH. They are combined back into da_lapse within the function.