Documentation: clearly state units of input of directional quantities

[GPMueller]

The docs should clearly state that everything is in Cartesian coordinates, such as

• external field
• anisotropy

unless specified otherwise, such as

• basis atom positions
• basis cell translations
• periodical boundaries
• spin polarisation
• temperature gradient

[GPMueller]

Implemented with b5d2464e00784a1a7a8a4c198fb9759dcab40b69.