Resolution of identity

[LinusBDittmer]

This branch implements the Resolution of Identity (RI, also density fitting / DF) approximation to spatial ERIs. This factors an ERI (pq | rs) into the product of either B^P_{pq} B^P_{rs} or V^P_{pq} (P | rs), which are called the symmetric and asymmetric decomposition modes. The following changes have been made:

• Added auxilliary indices as a new space with letters PQRSTUVWX
• Added 250 as a default size for RI indices in optimised contractions since X ~= O + V
• Fixed a bug in spin integration where an index that occurs twice in the same tensor (e. g. <ij || ij>) would spuriously cause an error
• Added tests for RI
• Completed the .gitignore to inlcude build-system files
• Changed CI to also run on branches other than master
• Fixed some spelling mistakes