Thick tracking in field-free dipole not consistent with tracking in drift

[chernals]

When tracking through a dipole with zero field (ie. ANGLE=0 and K1=0, K1S=0, etc.) the results are not the same as for a tracking through a regular drift.

This is due to the fact that the pure DRIFT uses the solution the full Hamiltonian (total momentum, including the square root); ttdrf in trrun.f90.

But, the bends use the truncated Hamiltonian (both using ttcfd in trrun.f90).

A correct implementation should detect that a generic combined function dipole (ttcfd) degenerates to a drift and switch to ttdrf. This is already done in ttquad when this happens in a quadrupole.

[tpersson]

I added a check for the 0 strength dipole in a similar way as for the quadrupole. This is in PR #846

[chernals]

Awesome, thanks.