Add 'extract' type-bound procedure to 'Colormap'.

[gha3mi]

Hello @vmagnin,

This PR introduces the extract feature discussed in #8.

With this functionality, it's now possible to extract colors from a colormap. The extracted number should be within the range [2, levels].

I compared some binary colormaps generated by the extract function with scientific colormaps, and they match.

Changes Made:

• Added the extract type-bound procedure to the Colormap type.
• Included an example (extract.f90) demonstrating how to utilize the 'extract' feature.
• Implemented two private functions: scale_real_real and scale_real_int.

Best, Ali

[gha3mi]

Maybe it is better now to update the COLORMAPS_LIST.md by removing the discrete colormaps.

[vmagnin]

Maybe it is better now to update the COLORMAPS_LIST.md by removing the discrete colormaps.

Yes, it would lighten the list, even if we let the colormaps in the Fortran module.

[vmagnin]

I think that in the example extract I will put 10 levels instead of 100, because with 100 we don't really see the difference from the original map (with 256 levels).

[gha3mi]

Yes, 100 were used in my last comparison. I forgot to decrease it.