[MRG + 1] Fix numerical instability in LassoLars when alpha=0 (#7778) (#7849)

* Fix bug 7778

* Add test_lasso_lars_vs_R_implementation

* Add a space to match the indentation

* Solve E501 line too long (80 > 79 characters)

* assert_array_almost_equal up to 12 decimals

* Tiny modification for increasing performance

* Update what's new page

* Trying to solve conflicts

* Solve conflict in doc/whats_new.rst