Merge branch 'removed_variable_shadowing' into 'master'

No shadowed variables

Some small improvements to the code:

 - Force compiler to report shadowed variables. 
 - Removed all shadowed variables. 
 - Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

This comes after me spending a day and a half tracking a bug that came from a shadowed variable.

See merge request !421