PointCloud interface consolidation and various optimizations

[fran6co]

This is a big change and I'm still working on it, but I wanted to get it out there as some other people are working on similar things.
This should supersede #1343 and #1340 and eventually fix #1306.

The end goal of this PR is to replace the std::vector structure of PointCloud to something else that doesn't have the same shortcomings. But there are other issues that this PR will fix:

• Most of the time width and height are updated manually, by using PointCloud interface the internal state is going to be always valid (Found several places where the PointClouds have invalid width and height)
• push_back is misused, most of the time the code knows the size of the PointCloud beforehand. push_back should be used only when the size of the PointCloud is not known before processing it
• PointCloud iteration use cases should be standardized, of course std::vector iterators should be removed and be replaced by PointCloud iterators that are compatible with stl. I have seen direct pointer access (&cloud->points[0]) that should be contemplated by the interface like OpenCV does (a ptr() method should be enough).

Take in account this is work in progress PR, I'm still splitting bigger commits in smaller ones so it can easily reviewed. I'm creating two types of commits: the big ones, that are just search & replace changes, and the rest that fix code or standardize it.

With all these changes points can be made protected (already tried and it compiles just fine).

[fran6co]

Things to do:

• There are still some places that use points directly but they need a custom patch
• The commit "Use PointCloud::operator [] instead of PointCloud::points::operator []" contains some other changes that should be split in additional commits
• Introduce ptr member to PointCloud and make the code that access the points by pointer use it
• Make the code that use begin and end from the PointCloud use CloudIterator

[fran6co]

@taketwo I think this can be merged and continue in a different PR the extra interface consolidation task. It's a pain to rebase it everytime something changes.

[stale]

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Come back whenever you have time. We look forward to your contribution.