altitude shift in flight altitude quantiles calculated by in integrate_profile()

[adokter]

Flight height quantiles (when using argument height_quantile) are incorrectly shifted down by up to one height bin. This issue has appeared after an earlier fix #532.

For example, for a radar with lowest available bin at 200 meter:

> example_vp$attributes$where$height
[1] 209

We find the lower end of the distribution starting at 100 meter instead of 200 meter:

> integrate_profile(example_vp, height_quantile=.00000001)
  radar            datetime      mtr      vid      vir      rtr mt rt       ff       dd         u
1 seang 2015-10-18 18:00:00 4108.289 89.08468 983.9208 45191.18 NA NA 12.81019 204.4764 -5.298934
          v height
1 -11.64013    100

[adokter]

This issue also gives rise to error message:

Error in if (is.na(delta_linear_interpolation)) delta_linear_interpolation = 0 : 
  argument is of length zero