Add Types to variables

[moritz155]

See presentation of Code Review 2 on Isis

[yunseopark-kaist]

https://www.youtube.com/watch?v=pvduxLowpPY

[yunseopark-kaist]

Before and after the modification at commit 9612416

*code for test:
ProfileView.@profview for i=1:10
parallelplot(DataFrame(height=160:180,weight=reverse(60:80),age=20:40),normalize=true)
end