Fix use of removed Datafile in Laplacian constructor; update docs

[ZedThree]

There was still a use of Datafile in the Laplacian constructor which one of the examples (hw-3D) was using, which meant it didn't compile.

I (finally!) added some docs on the new file IO system that relies on OptionsNetCDF, and how to update physics models to use it.

Also fix the predefined macros for doxygen so it preprocesses everything correctly.

Lastly, just for fun, I've updated the docs to use sphinx-book-theme as a refresh for v5. It's cleaner and easier to navigate than the default readthedocs theme.

This is (hopefully) truly the last PR for v5 -- everything else is getting bumped to 5.1, unless there are new build-blocking bugs discovered in the next week.

[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: constructor does not initialize these fields: jy, proc_in, proc_out, myproc, nproc, xs, xe [cppcoreguidelines-pro-type-member-init]

src/invert/laplace/impls/iterative_parallel_tri/iterative_parallel_tri.hxx:179:

-   int jy;
+   int jy{};

src/invert/laplace/impls/iterative_parallel_tri/iterative_parallel_tri.hxx:220:

-   int proc_in, proc_out;
+   int proc_in{}, proc_out{};

src/invert/laplace/impls/iterative_parallel_tri/iterative_parallel_tri.hxx:223:

-   int myproc;
+   int myproc{};

src/invert/laplace/impls/iterative_parallel_tri/iterative_parallel_tri.hxx:226:

-   int nproc;
+   int nproc{};

src/invert/laplace/impls/iterative_parallel_tri/iterative_parallel_tri.hxx:235:

-   int xs, xe;
+   int xs{}, xe{};

[github-actions]

warning: constructor does not initialize these fields: inbndry, outbndry [cppcoreguidelines-pro-type-member-init]

src/invert/laplace/impls/pcr/pcr.hxx:121:

-   int inbndry;
+   int inbndry{};

src/invert/laplace/impls/pcr/pcr.hxx:123:

-   int outbndry;
+   int outbndry{};

[ZedThree]

Here's how the new docs theme looks

Any major objections to the new theme?

[bendudson]

Thanks @ZedThree !