EPR calculation with PBC

[Reinout]

Commenting for the g-tensor only (but I assume most of it will apply
to the spin-spin coupling as well)

* Yes, please use the GAPW method
* Make sure to have all-electron basissets & potentials in the region
of the spin center (you can define GTH basissets & pseudopotentials
elsewhere)
* &XC_FUNCTIONAL blyp (or any other GGA functional) in section
&G_TENSOR requires additional implementation efforts. Use Xalpha
instead of blyp.

Further details to be found in the paper describing the NMR/EPR
implementation in cp2k: http://link.aip.org/link/?JCPSA6/131/014106/1

Reinout