implement alm2map_spin and map2alm_spin

`healpy` now switched to `libsharp`, `pshyt` is not built anymore so we have lost 2 functions:

* `alm2map_spin`
* `map2alm_spin`

there are 2 options for implementing them:

* go through healpix C++ and therefore modify the `_healpy_sph_transform_lib.cc`, this is not using `cython` but directly the python C++ API, quite messy to work with.
* wrap `libsharp` directly, taking as an example the `pshyt` wrapper that is still available in the pshyt folder, but not build by `healpy` as healpix now does not ship it anymore.

anybody willing to implement this?