Migrating all learning and experimenting material in a separate repo

[tlunet]

Following the discussion on #21, it would be better for the futur of qmat to keep it limited to its main use : generation of $Q$-coefficients and associated approximation.

Hence, all new features added with #21 and #20 will have to be moved to a separate repository / package,
with current name candidates :

• qlayground : qmat playground
• qmint : many flavors of time integration with qmat