add accelerator clock method

[BenjaminW3]

This adds the missing clock64() instruction returning a wall clock time in ticks which was noted in issue #18 as alpaka::time::clock(acc).

[erikzenker]

Why the renaming in this PR?

[BenjaminW3]

Because I wrote a new test kernel, looked for the current naming convention and found that this was the only test kernel that was named inconsistently 😄

[erikzenker]

Nice Work 👍 but you did also a lot of replacement of comments. Could you separate renamings and replacements which are unrelated to the time feature into a new pull request ?

[BenjaminW3]

Renamings, comment corrections and the time feature are in 3 independent commits, so merging it at once would not introduce any revert problems. However, if you insist on it, I could seperate them when I have time again.

[erikzenker]

It would be nice to split this PR up into a "new feature" and a "comment" PR. Because, if we search for some changes of code in the future it is easier to understand what happened in this PR.

[BenjaminW3]

The two other commits have been proposed independently as #177 and #178.

[erikzenker]

Thank you for separating this PR