4C bispinor bases

[lelemmen]

Describe the feature you'd like
In order to support Dirac-HF (DHF) calculations:

• we need a class that reflects a bispinor basis (i.e. a 4-component basis) in the code base;
• we have to be able to calculate relativistic integrals in this bispinor basis, see also Implement interface for more libcint integrals #10.

Describe what the current code offers in relation to what it lacks
There currently is no support for any kind of relativistic calculations, except for objects that can be real or complex. The current ˋSpinorBasesˋ are only suited for calculations up to 2 components (alpha and beta).

Additional context
In relativistic calculations, it is important that a kinetically/magnetically balanced bispinor basis is used. How could we implement a kinetically-balanced (or magnetically-balanced) basisset or bispinor basis?

A bispinor basis could have an L x L transformation matrix that is used to transform all components of the bispinor: either full 4C or as 2C.