enance
change the return type to float64 for Sum operation
remove the unused code
Revert "move two documents to wiki"

This reverts commit 9ec88d0b8508ebcd5df9fe76e8b4f9a5ea704aa1.
delete design.md since it be moved to docs
enhancement the performance of Sum
add MaxBy, MinBy, SumBy functions
1. remove MaxBy, MinBy, SumBy operations, them be merged to Max, Min, Sum operations.
2. add comparable interface
3. add Go Doc
split the linq.go to ling.go and operations.go
Merge branch 'develop'
Add optional convert parameter to Max, Min, Sum and Average operations