This PR moves around some functions from freqplot.py and timeplot.py to ctrlplot.py in preparation for an upcoming PR that is going to modify those functions. There is no change in functionality, but I want people to be able to see the changes I am making to functions in ctrlplot.py in the upcoming PR => need to move things around first so that git diff can highlight the changes.

Summary of changes:

• The rcParams dict used to customize plotting functions is now in ctrlplot.py (instead of having two separate but identical rcParams in freqplot.py and timpelot.py).
• The _process_ax_keyword utility function is now in ctrlplot.py (instead of freqplot.py).
• The _process_line_labels utility function is now in ctrlplot.py (instead of freqplot.py).
• The _get_line_labels utility function is now in ctrlplot.py (instead of freqplot.py).
• The _add_arrows_to_line2D utility function is now in ctrlplot.py (instead of freqplot.py).
• Rearranged some functions in grid.py.