UnimodalDataView.copy() returns MultimodalData by default

[yihming]

Fix 1

The copy() function of UnimodalDataView now returns a MultimodalData object by default (previously UnimodalData), with an option to_unidata if users still want a UnimodalData object.

Fix 2

pd.concat emits the following warning:

The behavior of DataFrame concatenation with empty or all-NA entries is deprecated. In a future version, this will no longer exclude empty or all-NA columns when determining the result dtypes. To retain the old behavior, exclude the relevant entries before the concat operation

when it concatenates at least one empty DataFrame. So add an justifier and explicitly set None for empty DataFrame objects.