Use inplace conversion

[Tortar]

This has some good impact on performance (something like 20% less time)

[Devetak]

Hi! Its not clear what the pos! function does since it is both in place and returns an array.

[Tortar]

I add some docs

[Devetak]

Woudn't be better if the function was either pure or in-place with no return. Looking at the use I would say

expression = pos_inplace(a / sum(a))
# transfor to
expression .= a / sum(a) # in - place 
pos_inplace!(expression)

[Tortar]

added some docs and put it next to the not-in-place version

[Tortar]

We need the return, otherwise we couldn't use it inside other computations

[Tortar]

it's also conventional in Julia (I think for this reason)