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Delete obsolete code and comment to make `get3DPoint` more understandable

d.kilic requested to merge 118-get3dpoint-2-methods-implemented-but-only-newer-used-2 into master

Restructuring of get3DPoint to be more easily understood. See the following survey comment for the used approach of the code.

Since we change from double to float mid-calculation, I couldn't rewrite everything as I wanted without breaking tests.

/* Basic Idea:
     * All points projecting onto a point on the image plane lie on the same
     * line (cf. pinhole camera model). We can determine this line in the form:
     *
     * g: x = lambda * v
     *
     * This line exists in camera coordinates. Let v be the projection with
     * depth 1 (i.e. v_3 = 1). Then lambda is the depth of the resulting point.
     * We'll continue to call lambda z instead, to show this.
     * We now want to determine the depth at which the resulting point has height h
     * in world coordinates. The transformation from cam to world is:
     *
     * W = R * C - T
     * W   := Point in World Coords
     * C   := Point in Cam Coords
     * R,T := Rotation and Translation of Cam
     *
     * By putting in our x = z * v, we get:
     *     W          = R * (z * v) - T
     * <=> W          = z * Rv - T
     * <=> W + T      = z * Rv
     * <=> (W + T)/Rv = z
     * We select the third row of this to solve for z. Finally g(z) is transformed
     * into World Coords.
     */

Closes #118 (closed)

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