Xobjects

**Changes:**
- Rename of several methods
- Use offset=0 as default of from_buffer.

**Changes:**

Improve HybridClass robustness:
- Correctly propagate xobjects and python attributes in assignment and initialisation within nested data structures.
- Prevent object move for nested objects and in the presence of references
- Referencing objects across buffers now results in an error.
- Fix in rename mechanism.

**Changes**

Improved and reorganised management of sources and dependencies:
- Replaced extra_sources and _extra_c_source with _extra_c_sources both in xobjects.Struct and in xobjects.HybridClass.
- Kernel declarations can be made through the _kernels dictionary when creating subclasses of xobjects.Struct and in xobjects.HybridClass. The kernels can be compiled by the compile_kernels(...) method.
- Dependencies on additional xobjects types can be provided through the _depends_on list when creating subclasses of xobjects.Struct and in xobjects.HybridClass. These are taken into account when compiling the kernels.
- Context.add_kernels can apply a user-provided list of functions to the source code before code specialization.
- Renamed HybridClass.XoStruct to HybridClass._XoStruct.
- Renamed HybridClass._move_to to HybridClass.move.

**Changes:**
- Removed dress function and renamed DressedStruct to HybridClass

**Change:**
- Disable OpenMP parallelization until it is optimized and properly tested

**New feature:**
- Possibility to skip kernel compilation (to access the complete source without compiling)