GS-70: Renaming the extension

Renaming the extension from uk.co.compucorp.civicrm.activityreport to uk.co.compucorp.civicrm.pivotreport.
Changing all in-code references from 'Activityrepot' to 'PivotReport'.
Changing namespace from CRM_PivotReport to CRM_PivotData.
Changing namespace from CRM_Activityreport to CRM_PivotReport.
Renaming files.
Removing 'default: Contribution' info from Report Selector remplate.
Updating info and readme files.

compucorp / uk.co.compucorp.civicrm.pivotreport