Support for inheriting sections

In the metainfo one section definition can extend another one through inheritance. A section definition defines other section definitions as base_sections. This is similar to class inheritance in programming. A sub section definition can denote a (potentially "abstract") section definition. When this is instantiated an inheriting section can be used for this sub section definition. This is similar to polymorphy in programming.

• simplify the programming of navigation across inheritance hierarchies with additional properties like all_base_section, inheriting_sections, and all_inheriting_sections (both in python and js)
• the metainfo browser shows inheriting sections
• the ELN editing in the archive browser allows to create sub-sections based on available inheriting sections