Add derive.param2.register

[proux01]

So that users can write

Elpi derive.param2.register foo foo_R

instead of

Elpi Accumulate derive Db derive.param2.db.
Elpi Accumulate derive.param2.db "
:before ""param:fail""
param {{ @foo }} {{ @foo }} {{ @foo_R }}.
".

[proux01]

After a rebase, CI green

[proux01]

@gares or @CohenCyril should this one be merged?

[proux01]

Thanks