Base class for data input processing modules, called "handler modules"
All modules that deal with processing input data extend from this class.
It provides access to input and state through the following member variables:
$session The session interface object
$request The HTTP request details object
$config The site config object
$user_config The user settings object for the current user
Modules that extend this class need to override the process function
Modules can pass information to the output modules using the out() and append() methods,
and see data from other modules with the get() method
Handler modules need to override this method to do work