add Gadget2Particle as it is. WIP - request for comments and help.

- Many routines translated from pynbody to read gadget2 files (format 1 and 2).
- Drastically reduced code repetition.

TODO:
- A config file needed for collect gadget user defined fields and relative units.
- StructArray data type (e.g. Float32 of Float64) can't be instantiated at runtime. I think it should be possible, but still I need to find a way.
- Rebase with latest modifications upstream

Author: elehcim

Project.toml

@@ -6,6 +6,7 @@ version = "0.1.0"
 [deps]
 BangBang = "198e06fe-97b7-11e9-32a5-e1d131e6ad66"
 CSV = "336ed68f-0bac-5ca0-87d4-7b16caf5d00b"
+Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"
 Distributed = "8ba89e20-285c-5b6f-9357-94700520ee1b"
 FileIO = "5789e2e9-d7fb-5bc7-8068-2c6fae9b9549"
 HDF5 = "f67ccb44-e63f-5c2f-98bd-6dc0ccc4ba2f"