Easy3D-v2.4.8

Main changes:

• introduced a new composite viewer class CompViewer, which supports multiple views arranged in a grid layout in the same window. See here for example usage.
• introduced usage_string_ and execute_func_, which eases the use of the viewer for running an algorithm.
• various minor bug fixes and improvements.

Pre-built binaries are not changed (i.e., you can still use the previous version.