Fixes for projected CRS alignment

[robe2037]

Explicitly sets normalized = TRUE in st_line_interpolate() to interpolate along full track lines when input data are in a projected CRS. Also coerces output of interpolation to sf to prevent mapply() from coercing to matrix and dropping CRS.

[robe2037]

Commit 2a53c0f is a proposed change to .ext(). This function had a few unused arguments and commented lines, so I wasn't completely sure of the intended scope of the function (e.g. if those lines were stale or intended to be implemented in the future). I removed them here to keep the code cleaner, but if I've misunderstood the overall goals and intended use of this function I can revisit those changes and make any further updates that are necessary.