CLN: avoid Unboundlocal error in tools/merge/_get_concatenaged_data (GH3833)
Merge pull request #3864 from jreback/concat_single_item

CLN: avoid Unboundlocal error in tools/merge/_get_concatenated_data (GH3833)
ENH/CLN: add figsize kwargs to hist method
DOC: release notes
TST: add test to check for passed figure not equal to passed ax's figure

This is only necessary for Series objects, since DataFrames use subplots which
ensures that the figure and axes are correctly matched.
CLN: use the figure object that was previously unused