Update to dag version with concurrent topological sorting

This release

1. Updates the fpm manifest to use version 2.1.0 of the Dag dependency 🌳 , which now features concurrent topological sorting 🎮 and
2. Replaces the Sourcery dependency with Assert, the new home of the assert subroutine that was previously in Sourcery. 🪄 🎩 🐰

Proof of concept

The example subdirectory in this release contains a first working proof of concept described in example/README.md. The example uses FEATS classes to traverse a directed acyclic graph that describes the module dependencies within FEATS itself. The motivating use cases is enabling parallel builds with the Fortran Package Manager. No actual compiling happens in this example, but the output indicates the order in which modules would be compiled and the images assigned to compile each module, submodule or main program.

Documentation: see the FEATS GitHub Pages site.