Let user specify datatypes of pandas dataframe in toPandas()

Would be nice to specify the dtypes of the pandas dataframe during the toPandas() call. Something like:

pdf = df.toPandas(dtypes={'a': 'float64', 'b': 'datetime64', 'c': 'bool', 'd': 'category'})

Since dtypes like `category` are more memory efficient, you could potentially load many more rows into a pandas dataframe with this option without running out of memory.