Add a postprocessor that returns the time step number

[anshchaube]

Motivation

This is a minor quality-of-improvement update to allow users to easily track the time step count during a MOOSE simulation.

Design

This information is already stored in _feproblem.timeStep();

Impact

Users will be able to track the time step number using a ready-to-use postprocessor, instead of relying on, say, a ParsedPostProcessor using TimePostprocessor and TimestepSize.