Third party PDF libraries must now be installed via composer and naturally
via composer autoloading mechanism. Because of that it is not necessary
to specify their path on disk. The usage is simplified and it allows us
to include them in our unit tests.
This also means that from now on PhpSpreadsheet must use composer autoloader
mechanism. The internal autoloading implementation was dropped.
This introduce a helper class that should be used to log things,
avoiding a lot of boilerplate code.
Also all output are made in /tmp folder instead of beside the script
itself. This is because there is a high chance that the folder containing
the script is not writtable by webserver. So using the /tmp folder
makes it more likely to works in a variety of setup.
Adrien Crivelli