Implementing stack() and concatenate()

This PR has been approved by @kaushikcfd and adds stack() and concatenate() which are borrowed from PyOpenCL. These features were added to help support the creation of a PyCuda interface in arraycontext

• Implemented gpuarray.(stack | concatenate)
• Implemented test_gpuarray.(test_stack | test_gpuarray)
• Updated array.rst
• Tested with flake8 and pytest