Suggestion: demo

[gha3mi]

• [ ] Move all test results to the demo folder to keep the main directory clean.
• [x] Add the write_ppm_file_colorbar subroutine as a type-bound procedure for the Colormap type. Move write_ppm_file_test to demo.f90 since it's a specific subroutine for the demo.

[vmagnin]

• Move all test results to the demo folder to keep the main directory clean.

Well, I let them in the root directory to avoid directories separator problems (see https://fortran-lang.discourse.group/t/how-to-access-a-subdirectory-in-a-win-unix-compatible-way/6630), but the discussion shown that it may not be a problem with recent MS Windows system. We could also use the fpm option that can be used to define the working directory (well, I tried and failed but maybe I did something wrong).

* Add the `write_ppm_file_colorbar` subroutine as a type-bound procedure for the `Colormap` type. 

Yes, the user can need it.

Move write_ppm_file_test to demo.f90 since it's a specific subroutine for the demo.

Yes, it seems logical.