Indexing and assignment for Hamiltonian

[pablosanjose]

Closes #24
Implements a variant of #22 (comment)

With this we can directly access the HamiltonianHarmonic matrix of h::Hamiltonian with h[n1, n2,...] where the n tuple is an integer cell distance (Harmonic vector, called dn in ELSA)

To add new (empty) harmonics do push!(h, dn). If it is already present, h will not be modified. (dn can be a tuple, a splatted tuple or an SVector)

To check if a harmonic is present do isassigned(h, dn)

To delete a harmonic do deleteat!(h, dn)

To assign an element i,j in harmonic dn do h[dn][i,j] = value

Broadcasting is allowed, so that h[dn...][is, js] .= value_matrix, to enable bulk assignment without intermediate allocations (at least if h is dense - a sparse matrix will typically require internal reshuffling to insert new elements).