Subaccess method for datatypes

Short description of the MR

Datatypes now have subaccess methods that return a pair of subtype pointer and subtype type.

List of things to check before making a MR

Before merging your code, please check the following:

• you have added a line describing your changes to the Changelog;
• you have added any new or changed yaml config format to the validation script;
• you have added unit tests for any new or improved feature;
• you have checked pdi/docs/CheckList.md
• you have checked your code format:
  • you have checked that you respect all conventions specified in CONTRIBUTING.md;
  • you have checked that the indentation and formating conforms to the docs/formating.astyle;
  • you have documented with doxygen any new or changed function / class;
• you have correctly updated the copyright headers:
  • your institution is in the copyright header of every file you (substantially) modified;
  • you have checked that the end-year of the copyright there is the current one;
• you have updated the AUTHORS file:
  • you have added yourself to the AUTHORS file;
  • if this is a new contribution, you have added it to the AUTHORS file;
• you have added everything to the user documentation:
  • any new CMake configuration option;
  • any change in the yaml config;
  • any change to the public or plugin API;
  • any other new or changed user-facing feature;
  • any change to the dependencies;
• you have correctly linked your MR to one or more issues:
  • your MR solves an identified issue;
  • your commit contain the Fix #issue keyword to autoclose the issue when merged;
• you have added to this file any other check that was missing.