Improve install and uninstall procedure Analytics API offered to plugins

Requirements Ability to install an old Moodle version and upgrade it Ability to edit the code prior the upgrade Core installation test Scenario: A fresh 3.7 installation has both in-built models installed. Install a fresh 3.7 site with the patch included Go to Site administration > Analytics > Analytics models TEST : Check that there are "No teaching" and "Students at risk of dropping out" enlisted TEST : Check that the "No teaching" model is enabled by default and the other one is disabled by default. Core upgrade test Scenario: Moodle 3.3 site upgraded to 3.7 has both in-built models installed. Install a fresh 3.3 site Upgrade it directly to the patched 3.7 site Go to Site administration > Analytics > Analytics models TEST : Check that there are "No teaching" and "Students at risk of dropping out" enlisted TEST : Check that the "No teaching" model is enabled by default and the other one is disabled by default. Plugin install test Scenario: Models declared by a plugin are created on plugin installation Install a fresh 3.7 site with the patch included Install the plugin https://github.com/mudrd8mz/moodle-local_testanalytics/tree/MDL-61667-37-demo (make sure to use this particular branch with the db/analytics.php file) Go to Site administration > Analytics > Analytics models TEST : Check that all the 6 models (e.g. "Linear", "Discreate", ...) declared by the plugin are created TEST : Check that out of those, only models aiming the example target "Binary", "Linear" and "Discrete" are enabled. All others like "Users without pic" are disabled even if they have a time-splitting method configured. Missing default models Scenario: Missing default models are automatically re-created Install a fresh 3.7 site with the patch included Go to Site administration > Analytics > Analytics models Delete the "No teaching" model Edit the main version.php and increase the version number Visit the admin notifications page to trigger the core upgrade Go to Site administration > Analytics > Analytics models TEST : Check that there the "No teaching" model has been re-created TEST : Check that the "No teaching" model is marked as enabled Customised default models Scenario: Customised default models aiming the same target are not duplicated Install a fresh 3.7 site with the patch included Go to Site administration > Analytics > Analytics models Edit the "Students at risk of dropping out" model Remove most of the selected indicators, keeping just a few of them Edit the main version.php and increase the version number Visit the admin notifications page to trigger the core upgrade Go to Site administration > Analytics > Analytics models TEST : Check that the "Students at risk of dropping out" model is there just once (no new model aiming the same target has been created) TEST : Check that the "Students at risk of dropping out" model has only the indicators that you left there after editing - the list of indicators was not reset to the default value.