Adds bwd_atan2().
Note that when y AND x contains zero, it is undefined, therefore not_implemented is called.
Adds first Unitary test.
Corrects mistake in bwd_atan2.xy
Adds unitary tests for bwd_add, bwd_sub, bwd_atan, bwd_sqrt.
Bugs found:
 - See results of unitary test bwd_atan04
 - See results of unitary test bwd_sqrt07
Corrects bwd_sqrt.
Fixes issue #87.
Adds unitary test bwd_atan2_02 as requested by @Jordan08 .
This test stills fails. Source of bug not yet discovered. It could be :
* comming from the tree based approach on building the bwd, or
* comming from the no respect of symmetry of bwd_atan.

Maybe @gchabert can enlighten us!
Comment update
Merge branch 'master' of https://github.com/msis/ibex-lib into msis-master
Merge branch 'msis-master' into develop