This refactors the system::default_device() method. Now, the
default compute device for the system is only found once and
stored in a static variable. This eliminates many redundant
calls to clGetPlatformIDs() and clGetDeviceIDs().
Also, the default_cpu_device() and default_gpu_device() methods
have been removed and their usages replaced with default_device().
This adds checks to the device test-suite to ensure that the
current device supports the partitioning types before attempting
to use the corresponding device::partition_*() methods.
This adds a new set of methods to the device class allowing
device objects to be partitioned into multiple sub-devices
using the clCreateSubDevices() function.
For now, device partitioning is only supported on systems
with OpenCL version 1.2 (or later).
This adds support for returning a std::vector<T> from the
various get_info<T>() methods. This provides a simpler
interface to get the values in an array returned from one
of the clGet*Info() functions.
This also adds a test using the new API to get the maximum
work item sizes in each dimension for a device.
