Modify the definition of "assistive technology" to align with the EAA definition

We are required to replace any standard-specific definitions with those that appear in the EAA. Currently EN 301 549 has its own definition for "assistive technology". We must now use the rather longer, more comprehensive EAA version:

any item, piece of equipment, service or product system including software that is used to increase, maintain, substitute or improve functional capabilities of persons with disabilities or for, alleviation and compensation of impairments, activity limitations or participation restrictions

This issue replaces the proposal in issue #156 (closed)