Interpolate a section of line
sec_to_interp(p1 = NULL, p2 = NULL, bd_r, loop_r = FALSE)
vector of length two giving the coordinates of the first point
vector length two giving the coordinates of the second point
matrix with two columns giving the fixed line on which to interpolate
boolean indicating whether the points are going in a loop
If only p1 is given the point is assumed to be the first in the sequence. If only p2 is given the point is assumed to be the last point in the sequence