This patch add a concrete subclass of vtkAbstractElectronicData
(vtkOpenQubeElectronicData) and a molecule source
(vtkOpenQubeMoleculeSource). These are compiled optionally only if
OpenQube is installed.

Eigen is needed to include OpenQube's headers.

Change-Id: I2b30d0d53ec9659b43adb7fd4dbe351de9d1d4b9