Add initial setup for pixi package manager

c++/.gitattributes

+# GitHub syntax highlighting
+pixi.lock linguist-language=YAML linguist-generated=true

c++/.gitignore

-/build
+/build
\ No newline at end of file
+# pixi environments
+.pixi
+*.egg-info

c++/pixi.toml

+[project]
+authors = ["Daniel Vedder <daniel.vedder@idiv.de>",
+           "Marco Matthies <71844+marcom@users.noreply.github.com>"]
+channels = ["conda-forge"]
+description = "A GUI frontend written in C++ for the Persefone.jl ecological simulator"
+name = "persefone-desktop"
+platforms = ["linux-64"]
+version = "0.1.0"
+
+[tasks]
+
+[dependencies]