BugFix: refpts=None in get_lifetime

[swhite2401]

PR #577 introduced a bug when refpts=None, this PR fixes this issue

[swhite2401]

@oscarxblanco sorry I overlooked the case where refpts=None in the previous PR, this one should fix the issue.

[oscarxblanco]

I have checked the call to get_uint32_index(at.All, endpoint=False) and it seems to work as expected for a given ring with 4043 elements

>>> len(ring)
4043
>>> ring.get_uint32_index(at.All, endpoint=False)
array([   0,    1,    2, ..., 4040, 4041, 4042], dtype=uint32)
>>> ring.get_uint32_index(at.All)
array([   0,    1,    2, ..., 4041, 4042, 4043], dtype=uint32)

When executing a test example, both options, with or w/o refpts work and write the expected warning message in the standard output refering to zero-length elements in the index list. This is normal behavior.

I was not able to finish the whole momentum aperture calculation due to some segmentation fault. But this seems to be not related to the indexing problem.

[swhite2401]

Ok so I propose to merge this as it fixes an obvious bug, on the other hand it would be good to understand this segmentation fault, I you have more details could you report it as a separate issue (it is most likely originating from the c part of the code).

I usually never run LT calculation for the full lattice but rather on a subset of elements (typically magnets) so it is well possible that for very heavy computations some problems have passed through the various validations.

[oscarxblanco]

@swhite2401 ok, I will report the segmentation fault in another issue.

From my side, this fix could be merged.