doxygen in config.h.in

element class modified: P matrix removed, computed when needed using …

…matrix block technique, element is 2*3*N*N double lighter

Fix sign of demagnetizing energy in the right place

The previous commit changed the sign of the contribution of the volume
charges to the demagnetizing energy. It turns out the sign within the
expression was right to begin with. However, it relied on
Tet::interpolation(), which had the wrong sign. This error did not
affect the magnetization dynamics because that particular overload of
Tet::interpolation() is only used to compute the demagnetizing energy.

For sanity's sake, use the right sign in both places.

CI: add Vagrant machine image ubuntu_noble

This is the current development branch of Ubuntu, to be released as
Ubuntu 24.04 LTS "Noble Numbat".

Fix the serialization of multiline functions

As both Bext and initial_magnetization can be defined as JavaScript
function expressions, this encourages the use of YAML multiline strings.
Let `feellgood --verify` properly serialize them as "literal style"
multiline strings, e.g.

    Bext: |
      function(t) {
          // ...
      }

Remove GMM as a dependency

GMM has now been completely replaced by Eigen.

Replace exprtk with Duktape

Duktape is a compact, easy to integrate embeddable JavaScript engine.[1]
Use it, instead of exprtk, to reimplement MagnetizationParser and
TimeDepFieldParser, without touching their interface. The expressions of
the initial magnetization provided by the user can be converted into
JavaScript function expressions by this string concatenation:

    "function(x,y,z) { return (" + expression + "); }"

Update the documentation files README.md and License.txt accordingly.
Also update install-dependencies.sh in order to install Duktape instead
of exprtk.

Tests show a spectacular reduction in build time and binary size. These
were measured by doing a clean build on an Intel Xeon W-2255 (10 cores,
20 threads), with `make -j 20`, no ccache, and a warm disk cache.
Duktape was linked statically in order to make the size comparison fair:

                            exprtk    duktape
        ─────────────────────────────────────
        build time (s)      52.257      5.316
        binary size     12,440,856  1,409,112

There is no measurable difference in feeLLGood's performance.

Note that the set of primitives available for building expressions has
changed: these are now interpreted according to the standard JavaScript
syntax, with the addition that all properties of the `Math` object can
be accessed without the `Math.` prefix. Worth noting:

 * π is `PI` (in upper case)

 * the exponentiation operator is `**`

 * the `random()` function returns a pseudo-random number within [0, 1)

[1] https://duktape.org/

mesh::save_vtk method removed

Add example shell script

This script simulates ferromagnetic resonance on a nanoparticle. It
illustrates how bash can be used to:

 - run multiple simulations in a loop

 - generate the settings file programmatically and feed it to
   feeLLGood's standard input

 - extract data from multiple simulations and consolidate it in a single
   file

 - use gnuplot to plot the consolidated data

Bump version number to 0.6.4

There have been 245 commits since 0.6.3.