Lepton included as a module

Inclusion is made using src/import.sh script.

cc: @peastman

This script manipulates a bit the source code to make it compatible
with PLUMED code base, namely:

- convert Lepton to lepton (needed to have namespace named `lepton` instead of `Lepton`).
- wrap in a PLMD namespace.
- add automatic headers.

The last step includes a COPYRIGHT header indicating the author of the library