Cleanup input parsing

Input parsing section cleaned up:

* input validator functions cleaned up
* `m2t.cmdOpts` is renamed to `ipp` for brevity and afterwards stored into `m2t.cmdOpts`
* a lot of superfluous/incorrect comments removed from the input parser section, only where the information is NOT covered by the help text of `matlab2tikz` were the comments retained.

This commit may conflict with pull requests #268, #284 and #286. It is advisable to first pull those and this one afterwards, the merging should then be quite straightforward as this pull request changes the order of some input arguments. I guess conflict resolution would then basically be renamed `m2t.cmdOpts` to `ipp` in the affected section (and possibly removing superfluous comments).