text: Title: Add tight layout option that enforces constant spacing between rows and columns
  Body: The ProPlot "tight layout" algorithm permits variable spacing between successive rows and columns. However, for the sake of symmetry/aesthetics, users may occaionally want constant spacing like in matplotlib.
  
  This will be trivial to implement in proplot. We can add keyword args (e.g. `wequal`, `hequal`, and `equal`) to the `Figure` class that force the tight layout algorithm to use the largest of the calculated spaces for each row-row/column-column interface.
  