Create object containing two-body elements as blocks

[Schoyen]

Several ab initio methods are tailored to use only parts of the two-body elements. For example, the faster methods often avoid the pppp-block of the two-body elements. To reduce the memory load and allow for larger systems to be run we should store the two-body elements as blocks where we can optionally choose to only populate the necessary blocks. An abstraction to this (avoiding the need to rewrite a lot of downstream code) is to create an object containing the two-body elements as blocks and implementing the __getitem__-dunder method such that we can pass in slices and index tuples similar to NumPy-arrays. The method should then return the relevant elements/blocks.