Adding new exercises

As discussed on the [Forum](https://forum.exercism.org/t/fortran-track-add-new-exercises/63295/13), the Fortran track currently has 35 practice exercises implemented, 103 unimplemented. This makes it one of the smallest of the 83 tracks.

Personally, I can't get excited about the string-building exercises in Fortran, such as `bottle-song`, `food-chain` or `house`. They are possible to add, but far from the numerical calculations that Fortran is normally used for.

Quite a few other exercise types are (at best) difficult with the minimal installation used in the test runner. In Fortran, `stdlib` is not standard: it is a community project that needs separate installation. This steers us away from date/time exercises like `meetup` and `swift-scheduling`.

Nested arrays are abnormal in Fortran (though you can declare a derived type containing an allocatable array component), so `flatten-array` would be strange.

More promisingly, there are various categories that might suit the track, though with more careful thought needed.

- General numerical exercises such as `all-your-base`, `largest-series-product`, `prime-factors`, `pythagorean-triplet`.
- Bit-twiddling, as in `eliuds-eggs` and `secret-handshake`.
- Some of the various cipher exercises.
- Exercises with 2-D boards, treated as a matrix: `game-of-life`, `flower-field`, `spiral-matrix`.
- Parallel execution, as in `parallel-letter-frequency`.

Start with the linked Forum discussion and review the proposed exercise categories, Fortran's minimal test-runner environment, and the existing 35 implemented exercises. No repository files or tests are named in the issue. Done requires an agreed set of suitable exercises and their implementation in the track, with the relevant tests passing.

Written by the indexing model from the issue text.