Correct angle_max to avoid fencepost error

[kw90]

The current r2000_node.cpp creates scans within the range of (-π,+π) with a dynamic angle_increment that is defined by the samples_per_scan variable. Using this range leads to a fencepost error, where the scan in 0° direction is contained two times and therefore seems to be missing.

Some ROS libraries (e.g. navigation_2d) complain about that error and others ignore it. Ignoring it leads to a incorrect conversion of the scan points into 2D coordinates (which is important for accurate SLAM).

Since the deviation is neglible and not necessarily visible (and thus mostly ignored), it can be further rectified by adapting the maximum range of the angle:

$$angle_min = -π angle_max = +π-angle_increment$$

Thank you for considering merging this change. :)