Description

In preparation for using headers when building C tracking code:

• introduce context based defined flags: XO_CONTEXT_CPU, XO_CONTEXT_CPU_SERIAL, XO_CONTEXT_CPU_OPENMP, XO_CONTEXT_CUDA, XO_CONTEXT_CL.
• add extra_compile_args parameter to Context.add_kernels and propagate it correctly

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts