Properly comment qmckl_probes

Add comment to explicit the use of qmckl_probes and provide a link to
Verificarlo's documentation

tools/qmckl_probes.c

@@ -8,8 +8,31 @@ vfc_probes probes;
 #endif
 
 
-// Wrappers to Verificarlo functions
+// QMCkl is a wrapper to Verificarlo's vfc_probes system. The goal of QMCkl 
+// probes isto simplify the use of vfc_probes, and to provide functions that 
+// can be called either wit or without vfc_ci support by using #ifndef 
+// statements : 
+//
+// - when vfc_ci is disabled, qmckl_probes functions will either return false 
+// (no error) or perform a check based on a reference value
+// - when vfc_ci is enabled, qmckl_probe functions will simply encapsulate 
+// calls to vfc_probe 
+//
+// Moreover, one does not have to worry about the life cycle of the probes 
+// structure, as it is automatically created, dumped and freed by this wrapper.
+//
+// vfc_ci support can be enabled by using the following configure command : 
+// QMCKL_DEVEL=1 ./configure --prefix=$PWD/_install --enable-silent-rules 
+// --enable-maintainer-mode CC=verificarlo-f FC=verificarlo-f --host=x86_64
+// 
+// Finally, this wrapper also comes with a Fortran interface (in its dedicated 
+// file).
+//
+// To learn more about Verificarlo CI : 
+// https://github.com/verificarlo/verificarlo/blob/master/doc/06-Postprocessing.md#verificarlo-ci
 
+
+// Automatically initialize the vfc_probe object if VFC_CI is defined
 #ifdef VFC_CI
 void __attribute__((constructor)) qmckl_init_probes(){
 	probes = vfc_init_probes();
@@ -17,6 +40,9 @@ void __attribute__((constructor)) qmckl_init_probes(){
 #endif
 
 
+// Standard probe, without check
+// - if VFC_CI is defined, place a standard probe
+// - if VFC_CI is undefined, return false (no error)
 bool qmckl_probe(
     char * testName,
     char * varName,
@@ -30,6 +56,10 @@ bool qmckl_probe(
 }
 
 
+// Probe with absolute check
+// - if VFC_CI is defined, place a probe with an absolute check
+// - if VFC_CI is undefined, perform an absolute check based on target value
+// and accuracy
 bool qmckl_probe_check(
     char * testName,
     char * varName,
@@ -45,6 +75,10 @@ bool qmckl_probe_check(
 }
 
 
+// Probe with relative check
+// - if VFC_CI is defined, place a probe with a relative check
+// - if VFC_CI is undefined, perform a relative check based on target value
+// and accuracy
 bool qmckl_probe_check_relative (
     char * testName,
     char * varName,
@@ -59,6 +93,8 @@ bool qmckl_probe_check_relative (
 #endif
 }
 
+
+// Automatically delete and dump the vfc_probe object if VFC_CI is defined
 #ifdef VFC_CI
 void __attribute__((destructor)) qmckl_dump_probes(){
     vfc_dump_probes(&probes);