template<int depos_order>
struct Compute_shape_factor< depos_order >
Compute shape factor and return index of leftmost cell where particle writes. Specializations are defined for orders 0 to 4 (using "if constexpr"). Shape factor functors may be evaluated with double arguments in current deposition to ensure that current deposited by particles that move only a small distance is still resolved. Without this safeguard, single and double precision versions can give disagreeing results in the time evolution for some problem setups.