Add python bindings

[ll-nick]

As promised, here is a taste of what's to come :)

This contributes python bindings. Since this is a templated library, the python bindings have to be compiled at a later point in time, when the types are known. Therefore, we don't ship the bindings directly but rather add a convenience function that allows binding the types once the template arguments are known.

Now util_caching brings some extra challenges because it is possible to use it with very different kinds of keys. I solved it by adding two binding functions. One to bind a types you need for a number based key and an analogous one for time based keys.

When pybind is available and BUILD_TEST=TRUE, a python module is built that allows running unit tests (almost) equivalent to the currently implemented ones.

[EDIT] The following is outdated and has been fixed, see comment below.

There are two things missing right now. This line as well as this test.

The first one would require calling

python_api::bindTimeBasedCache<Time, double, std::chrono::seconds>(timeBasedSeconds);

Since these binds some of the same types as the existing statement, the resulting modules can't be loaded in the same python file.

Similarly, adding the SomePolicyWithoutParams would require a new module.

We could add those but in my opinion it would just add quite an overhead for a very small gain therefore I chose not to add it but we can of course discuss that.

[ll-nick]

One idea I had one the weekend which might be a nice solution for allowing to add more comparison policies then the already implemented ones would be to add a template parameter pack and running

        .def(
            "cached",
            [](CacheT& self, const NumberT& key, const ApproximateNumberT& policy) {
                return self.template cached<ApproximateNumberT>(key, policy);
            },

for each passed policy. I'll set this PR back to draft and test that out first as it would solve the issue with the missing tests (I think).

[ll-nick]

Ok, that actually worked out quite well.

I rearranged the bindings a little and added the missing unit tests. It is now possible to add a comparison policy as a template parameter to the bindCache function which will the overload the cached function for it. Check the comments in the code for a detailed explanation, I tried documenting all of it :)

[ll-nick]

Rebased on current master.

[ll-nick]

Rebased to main and adjusted to updated cmake structure.

[ll-nick]

Rebased to main