double check cell subtype distribution implementations

[mgeplf]

It was mentioned here that:

I will propose a fix in my next commit and double check every over cell subtype distribution implementations.

[drodarie]

I double checked the implementations for cell subtype density placement, and they actually produce correct results.
I got confused because the implementation in atlas-densities and my previous one are different.The difference is that the old one was also used to place the cells while the new one only focus on computing celltype densities.
For the purpose of atlas-densities, this is sufficient, so we can close this issue.

[mgeplf]

Ok, sounds good - thanks for looking.