PHP 8 Support. Minimum version bumped to PHP 7.2+

• PHP 8 Support. Minimum version bumped to PHP 7.2+
• #105 address long existing memory leak when batch generating several graphs
• #103 fixes #88 "Trying to access array offset on value of type null" for CSIM plots.
• #102 #101 remove dev related stuff from autoloader, and examples/tests from exported tarball
• #96 enables led examples which didn't have autoloader's logic yet
• #100 handles #99 (PHP8's way to handle invalid GD image resource)
• #93 handles #92 , a misleading exception message when trying to render invalid text with GD