fixed bug of sessions only having one ascent
showing session table in statistics

showing number of ascents in chart
refactored stored grades in Boulder & User

- using class TentativeGrade (which encapsulates mean and variance)
- moved logic for grade calculations to GradeLearningService
restrict ascents taken into account for user grade calculation

only use ascents where
- user did not flash boulder easier than his ability
- user did flash boulder harder than his ability