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Make OpenCL DeviceVendor into enum class and move to GPU traits

The device context in OpenCL requires the information on vendor when
constructed. To prepare for opaque DeviceContext, the vendor
enum was moved into OpenCL traits.

Refs. #3311, needed for #3315.

Change-Id: Iec22ff17543b6a99407048de6e0cd82bb7218fb0

Move DeviceInfo into GPU traits

The DeviceInfo is needed upon construction of DeviceContext. To
prepare for opaque DeviceContext type, it is moved to GPU traits
and renamed according to the common naming scheme.

Refs. #3311, needed for #3315.

Change-Id: I2a9f1d932f142d645df75901521a734d208de509

Make DeviceContext into a proper class

Having DeviceContext as a platform-agnostic class allows one
to easily create context, which is usefull for setting up the
GPU stream/context manager, tests for GPU functionality.

Refs. #3315.

Change-Id: I496d225b58c96d7642830c8e9552139750891849

Use init(..) function to build DeviceContext

This patch unifies the logic of OpenCL context creation in PME and
NBNXM by using the same init(..) function for the DeviceContext
object.

Also, the DeviceInfo is now de-referenced directly after the check
on the pointer validity and passed along as a const reference, which
improves the clarity of the code.

Refs. #3315, #3311.

Change-Id: I5ba0f530918f3340fa1a5ad3e8d60fe4e0967dab

Take over management of OpenCL context from PME and NBNXM

This patch set creates the DeviceContext in runner and passes it to the
consumers (PME and NBNXM). This removes unnessesary management code
duplication, makes the device buffers in two modules compatible.

Fixes #2522
Fixes #3315
Refs #3311

Change-Id: I10358cfaced5b5c7dbdddf95679c9a9703f3a2c0