Cache self energies in memory.
In each step of the SCF loop of a device calculation, the same self energies need to be evaluated.
SaveInMemory() storage strategy specifies that each
self energy is calculated only once and stored in memory.
The self energies are reused in subsequent iterations.
This strategy can significantly improve performance,
but requires additional memory.
StoreOnDisk() strategy provides a similar performance improvement
without the additional memory requirements.