Sign of pitch (NED to ENU)

[xyp8023]

I think there is an issue of the sign of the pitch basically parsing every data format, since marine people all use NED convention, and in auvlib we use ENU. The heading should already be correct, and the roll is the same, discussion see ISSUE #69

Hi Nils,

I just realized that we never fixed https://github.com/nilsbore/auvlib/issues/69 by either flipping the sign of pitch or switching to NED convention in auvlib. So I created this PR, hoping to get merged to master.