API change: bye bye conditionals (#1225)

Description: Removes conditionals from the API. On the multi_stage level it is easier to implement them by multiple computations.

API change: conditionals (`if_` and `switch_`) are removed.

Author: anstaf

docs_src/manuals/gridtools_lexer.py

@@ -45,10 +45,7 @@
         'call',
         'call_proc',
         'with',
-        'at',
-        'if_',
-        'switch_',
-        'case_'
+        'at'
 ))
 
 gridtools_namespace = ((

docs_src/manuals/user_manual/advanced_functionality.hrst

@@ -3,6 +3,5 @@ Advanced Functionality
 
 .. include:: stencil_functions.hrst
 .. include:: software_caches.hrst
-.. include:: conditional_switches.hrst
 .. include:: expandable_parameters.hrst
 .. include:: global_accessor.hrst