Implement named structure in numpy and implement them for GEAR and SPHENIX

Fix #747 (closed) and #737 (closed)

SPHENIX was not working anymore because we were defining far too many masks in the hydro module. I am fixing this by merging some masks together. When we merge them, we might end up with data that are not homogeneous (e.g. float != int) and, in the current implementation, it was not really possible to deal with them. Therefore I am also adding the new function logger_loader_python_field_add_subfield that allows to define each field in a grouped mask. With this, I am constructing an hybrid dtype where each sub-element is named and can be accessed directly by its name.

The important work is done in logger/logger_python_tools.h and logger/logger_python_wrapper.c. Everything else is just redesigning the outputs.

logger/chemistry/GEAR/logger_chemistry.h

@@ -147,7 +147,16 @@ chemistry_logger_interpolate_field_spart(
 __attribute__((always_inline)) INLINE static void
 chemistry_logger_generate_python_part(struct logger_python_field *fields) {
   fields[chemistry_logger_field_part_all] =
-      logger_loader_python_field(2 * GEAR_CHEMISTRY_ELEMENT_COUNT, NPY_DOUBLE);
+      logger_loader_python_field(/* Dimension */ 2, CUSTOM_NPY_TYPE);
+
+  logger_loader_python_field_add_subfield(
+      &fields[chemistry_logger_field_part_all], "SmoothedMetalMassFractions",
+      /* offset */ 0, GEAR_CHEMISTRY_ELEMENT_COUNT, NPY_DOUBLE);
+
+  logger_loader_python_field_add_subfield(
+      &fields[chemistry_logger_field_part_all], "MetalMassFractions",
+      GEAR_CHEMISTRY_ELEMENT_COUNT * sizeof(double),
+      GEAR_CHEMISTRY_ELEMENT_COUNT, NPY_DOUBLE);
 }
 
 /**