Fix exporting of extra columns to LAS

[gillins]

Original report by Sam Gillingham (Bitbucket: gillins, GitHub: gillins).

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Currently when exporting to LAS, if there are columns that aren't part of the LAS spec they get saved as extra columns or 'attributes'. However, when reading the file back in with PyLidar you get a segmentation fault. Changing the version to LAS 1.4 seems to help a bit but other errors emerge.

Seems that using LAStools for this isn't easy. I've tried a number of things, but we need someone who has experience of the library to give us some assistance.