Xobjects version 0.1.21

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.