Allocation testing for Hamiltonians

[joachimbrand]

For the efficient performance of FCIQMC or matrix-free exact diagonalisation it is essential that the computation of matrix elements is done efficiently and without incurring heap allocations, as these calculations have to be performed within hot loops.

The functions provided in the module Rimu.InterfaceTests already provide basic tests to check that the interfaces for AbstractHamiltonian, AbstractOperator and AbstractObservable are correctly implemented. It would be useful for these functions to also test that the interface methods are allocation free.

This could be achieved by making use of AllocCheck.CheckAllocs, Test.@inferred and @allocated.