I want to implent logistic chests to be mergeable. Only chests of same type are mergable.

As I looked through your code this breakes down to following tasks:
1. Add "chest_type" parameter through all the code. So all existing entities would be "steel". Would you accept pull-request to your code if I do that?
2. Add some graphic files - they would also be typed: wooden, iron, steel, +5 logistic types. I can do that too.

Also I don't know if I have enough time to implement both. Should I start with 2 or 1?

1. I think this is already done, since the mod already supports merging of multiple types of chests with each other by grouping selected chests by name. The new chest names would just have to be added to the "MergingChests.MergableChestIdToData" map. I'm just not sure if logicstic chests have some other specific functionality which will need to be solved in control.lua.
2. Is definitely the larger problem, since logistic chests should probably have some animation to handle the bots, so it might be a problem with segmented sprite this mod is using.

If you want to take care of this, then start with 2. Unfortunately I don't have this mod in any remote VCS, so if you could upload the image segments (preferably with the coordinates data from prototypes/sprint-segments.lua) either here or send me an email to tomas.chmelik2@gmail.com.

I'm still sure it should not be part of base mod, but it would be handled very nicely as another marker mod which enabled new feature in main mod.

Thank you in advance