vikunja acc object for specific device funcitons

[SimeonEhrig]

Alpaka provides different device function via the alpaka acc to access the current id (alpaka::getIdx), do math (alpaka::math::abs) and other things. Some of the functions allows to break the constrains of the vikunja algorithm like writing to a specific element via thread id in the input memory. Therefore @j-stephan suggest in #59 (comment) to provide an vikunja::acc type which only allows access to safe functions.

This issue should collect feedback and should decide if it is useful to implement a vikunja::acc object.

Pros

• make it more safe the acc object

Con

• extra work to maintain the vikunja::acc object and keep track with the alpaka::acc object
• for some functions, it is not clear, if they should be forbidden or not