Fixes Fortran API output routines

The jagged vector outputs i.e. list[list[tuple]], etc. where incorrectly unpacked from C to Fortran. This PR addresses these indexing errors.

It also fixes a logic error in tutorial 18, where removing entities from a 2D array was done incorrectly.

With these changes I have been able to setup locally a CI pipeline that compiles and runs the examples, with all runtime checks enabled (-fcheck=all). The CI will be hosted at the fpm version of the Fortran API repo (https://github.com/gnikit/gmsh-fpm/), the PR to watch is (https://github.com/gnikit/gmsh-fpm/pull/36). In the future, I will see how we can include the example compilation and execution as part of the testing suite.

P.S. there are still some non-critical bugs with the interactive tutorials like t13, but that's a problem for future me.