Add qmckl_probes description to the code documentation

org/qmckl.org

@@ -256,6 +256,64 @@ cppcheck --addon=cert --enable=all *.c &> cppcheck.out
    functions  will  use  the  precision specified  in  the =context=
    variable.
 
+   In order to automatize numerical accuracy tests, QMCkl uses 
+   [[https://github.com/verificarlo/verificarlo][Verificarlo]] and 
+   its CI functionality. You can read Verificarlo CI's documentation 
+   at the [[https://github.com/verificarlo/verificarlo/blob/master/doc/06-Postprocessing.md#verificarlo-ci][following link]].
+   Reading it is advised to understand the remainder of this section.
+   
+   To enable support for Verificarlo CI tests when building the 
+   library, use the following configure command : 
+
+   #+begin_src bash
+   QMCKL_DEVEL=1 ./configure --prefix=$PWD/_install --enable-silent-rules --enable-maintainer-mode CC=verificarlo-f FC=verificarlo-f --host=x86_64 --enable-vfc_ci
+   #+end_src
+
+   Note that this does require an install of Verificarlo *with 
+   Fortran support*. Enabling the support for CI will define the 
+ ~VFC_CI~ preprocessor variable which use will be explained now.
+
+   As explained in the documentation, Verificarlo CI uses a probes 
+   system to export variables from test programs to the tools itself.
+   To make tests easier to use, QMCkl has its own interface to the 
+   probes system. To make the system very easy to use, these functions 
+   are always defined, but will behave differently depending on the 
+ ~VFC_CI~ preprocessor variable. There are 3 functions at your 
+   disposal. When the CI is enabled, they will place a ~vfc_ci~ probe 
+   as if calling ~vfc_probes~ directly. Otherwise, they will either do 
+   nothing or perform a check on the tested value and return its result 
+   as a boolean that you are free to use or ignore. 
+   Here are these 3 functions : 
+
+   - ~qmckl_probe~ : place a normal probe witout any check. Won't do 
+   anything when ~vfc_ci~ is disabled. 
+   - ~qmckl_probe_check~ : place a probe with a relative check. If 
+ ~vfc_ci~ is disabled, this will return the result of a relative 
+   check (|val - ref| / ref < accuracy target?). 
+   - ~qmckl_probe_check~ : place a probe with an absolute check. If 
+ ~vfc_ci~ is disabled, this will return the result of an absolute 
+   check (|val - ref| < accuracy target ?)
+
+  If you need more details on these functions or their Fortran 
+  interfaces, have a look at the ~tools/qmckl_probes~ files.
+
+  Finally, if you need to add a QMCkl kernel to the CI tests 
+  or modify an existing one, you should pay attention to the 
+  following points : 
+
+  - you should add the new kernel to the ~vfc_tests_config.json~ file, 
+  which controls the backends and repetitions for each executable. More 
+  details can be found in the ~vfc_ci~ documentation. 
+
+  - in order to call the ~qmckl_probes~ functions from Fortran, import 
+  the ~qmckl_probes_f~ module. All Fortran functions are appended with 
+  the ~_f~ suffix.
+
+  - if your tests include some asserts that rely on accurate FP values, 
+  you should probably wrap them inside a ~#ifndef VFC_CI~ statement, as 
+  the asserts would otherwise risk to fail when executed with the 
+  different Verificarlo backends.
+
 ** Algorithms
 
    Reducing the scaling of an  algorithm usually implies also reducing