API Refactoring and Documentation

[jjfumero]

Description

API Refactoring (no need for loaders of the Device specific implementations) and documentation for the Backend API. Major list of changes:

• Javadoc for the Backend API (TornadoBackend, Policy, WorkerGrid
• Cleanup unused methods
• Rename TornadoRuntime to TornadoRuntimeProvider
• Rename TornadoRuntimeInterface to TornadoRuntime
• Rename TornadoSettingsInterface to TornadoSettings
• Remove loaders for device-specific backends. There is no need to have this anymore.

Problem description

If the patch provides a fix for a bug, please describe what was the issue and how to reproduce the issue.

Backend/s tested

Mark the backends affected by this PR.

• [X] OpenCL
• [X] PTX
• [X] SPIRV

OS tested

Mark the OS where this PR is tested.

• [X] Linux
• [ ] OSx
• [ ] Windows

Did you check on FPGAs?

If it is applicable, check your changes on FPGAs.

• [ ] Yes
• [X] No

How to test the new patch?

make BACKEND=spirv,opencl,ptx
make tests

[jjfumero]

Thanks @stratika . All changes done.

[stratika]

Perfect! LGTM, let's merge first #456 in order to try this PR also with Ray-Tracer, before merging with develop.