implemented comments on my previous pull request (unified thick tracking for combined function dipole)

[alatina]

• changed subroutine's title ("cosmetic" change)
• changed declaration of double precision complex to conform to other parts of the code

[coveralls]

Coverage remained the same at 66.797% when pulling 75656f0 on alatina:master into 1a64dca on MethodicalAcceleratorDesign:master.

[ldeniau]

As discussed, it would be better to restore the ttquad map for pure quadrupole and use ttcfd as a combined sbend only. It will make the quad simpler and faster, and available for crosscheck with ttcfd(k0=0)!

[ldeniau]

intermediate state approved.