Implement reading measures

[dkrako]

Description of the problem

Having AOIs and fixations available, reading measures can be computed.

Description of a solution

• [ ] define list of reading measure methods
• [ ] implement list of reading measure methods
• [ ] create tests and make them pass

Minimum acceptance criteria

• [ ] reading measure implementations with passing tests

[SiQube]

a first step was done in #784 and #804