Xobjects

**New feature:**
- Introduced linked array (custom __setitem__)

**Fix:**
- missing _size in nested structs

**Fix:**
- Avoid dependency on cupy and pyopencl on package import

**New feature**
- to_dict now performs by default a copy to CPU

**New features:**
- dress functionality moved here from xtrack (included new features _reinit_from_xobject and _move_to).
- Possibility to dress classes by inheriting from DressedStruct

**New feature:**
- Source code is attached to kernel objects for inspection