Fix SWIG methods that return char**.

+ Add StringArray class to manage and manipulate arrays of fixed-length strings.

  This class is now used to wrap any char** parameters, manage memory and
  manipulate the strings.

+ Fix SWIG LGBM_BoosterGetEvalNames call that would result in a segfault.

  Added wrapper LGBM_BoosterGetEvalNamesSWIG (the method it wraps didn't work)

+ For consistency, LGBM_BoosterGetEvalNames was wrapped as well.

  Re-factored wrapper LGBM_BoosterGetEvalNames to follow LGBM_*
  convention of 0/-1 in case of success/failure.

swig/char_array_helpers.i → swig/StringArray_API_extensions.i

@@ -9,9 +9,9 @@
  * them.
  *
  * It also implements working wrappers to:
- *  -
- *  - LGBM_BoosterGetFeatureNames (original hidden and replaced with a ...SWIG version).
- *
+ *  - LGBM_BoosterGetEvalNames    (re-implemented with new API)
+ *  - LGBM_BoosterGetFeatureNames (original non-wrapped version didn't work).
+ * where the wrappers names end with "SWIG".
  */
 
 // Use SWIG's `various.i` to get a String[] directly in one call:

swig/lightgbmlib.i

@@ -292,4 +292,4 @@ TYPE *NAME##_handle();
 /* Allow retrieving handle to void** */
 %pointer_handle(void*, voidpp)
 
-%include "char_array_helpers.i"
+%include "StringArray_API_extensions.i"