Proofs of my_theta_is_basis (WIP), interp_op_local_proj and sigma_of_interp_op_local.
Add results on interp_op_local (WIP).

Add swap of two variables.
Add extensionality result on bigop.
Add def of span + some results.
Add def of gather + some results.
Add def of is_dual_family + some results.

Prove interp_op_local_is_poly lemma.
Some issues of mult/scal in kronecker.v

Create a new file of bijectivity with all necessary lemmas.

ajouter des commentaires pour faciliter la lecture.
déplacer des morceaux en sandbox.

Rd = 'I_d -> R.
+ autres...

prove theta_shape_fun_local lemma.
Re-define the FE of reference.
Formalize elt_geom with bigop and 'I_n.
displace some parts to sandbox as we don't need them.

- Some cleaning in finite_element.v.