Change cbar_short_side_pad to cbar_length

[spencerkclark]

cbar_short_side_pad is a mouthful of an argument name; it is also somewhat of an odd measure (denoting the distance between the ends of the colorbar and the edges of the plot area or figure area depending on the cbar_mode). In an offline conversation with @nathanieltarshish, it was determined that it may make more sense (when paired with cbar_thickness) to allow the user to specify the length of the colorbar.

It would also be nice if one could specify this measure in either absolute or relative space. For the colorbar thickness (there called cbar_size), this is enabled in AxesGrid by allowing the user to specify either a float (for specifying a measure in absolute space) or string of the form '5%' (for specifying a measure in relative space). We could adopt a similar convention here.