Add .pixi to .gitignore

[tud-mchen6]

If .pixi is not added to the global .gitignore on the user's end, dealing with .pixi might be a bit cumbersome. Adding it to the template of the module will preclude this issue. (Otherwise, deleting .pixi results in 999+ changes...)

[sjpfenninger]

Will be fixed via modelblocks-org/data-module-template#29