Extension: Geometries

This extension adds new features to the FELTOR core dg library.

• This header incurs a dependency on jsoncpp (or nlohmann::json if DG_USE_JSONHPP is defined)
• several grid generator classes are added, among them our new Hector class are added to the dg::geo namespace
• a bunch of new functors implementing various magnetic field geometries and profiles are added to the dg::geo namespace
• there are some miscellaneous additions like a flux surface average class and one used to integrate the field lines for parallel derivatives all in the dg::geo namespace.

We have a collection of writeups:

• The parallel derivative
• The HECTOR algorithm