suggested option and documentation additions to plot functions

[zabore]

I noticed a few small things about the plotting functions and their associated documentation that I thought could be improved, and am proposing the following changes:

• The plot_pep() and plot_map() function documentation indicates low, high, and mid color options, but I believe the functions have changed in the meantime to accept a palette argument instead. I updated the documentation accordingly.
• I also found it difficult to see the numbers on the plots produced by plot_pep() and plot_map() with the new default color palette, which results in white font on very light background in some places, so suggested adding options text_color and tile_color so that the user can change both the text color and associated tile color if desired. Both still default to "white" as before.
• The documentation for both plot_pep() and plot_map(), including the examples, indicated that the full object produced by a call to mem_exact() or mem_mcmc() was passed to these functions, but in fact they take the basket element of the object. I updated the input description as well as the examples correspondingly.
• I don't think the probability legend makes sense on the MAP plot produced by plot_map() so I suggest removing it entirely
• I changed the title size on the plot produced by plot_map() to be the same as that produced by plot_pep() so that the resulting plots could more easily be placed side-by-side.