Allow summarize to work with dates

[hofnerb]

e.g. via summarize_numeric()

[hofnerb]

In principle it works. However, the date class is lost when results are written into one combined data set. The problem is especially hard to solve if we want to mix numerics and dates in one summary.

• The only solution I can currently think of is to coerce the results to characters.
• Alternatively, one could use another format for the results, i.e., not a data.frame but a matrix type format where every cell has its own class. Is this possible?