Ensure most_common returns correct dtype

[BSchilperoort]

Fixes #52

I had forgotten to expose fill_value. Additionally, the .where call in restore_properties casts the data to NaN even when the data can cover the target grid.

With this PR, when using most_common:

• if the grid can be covered the dtype should stay the same
• if the grid cannot be covered and fill_value is None, the result will be cast to float and the user is warned
• if the grid cannot be covered and fill_value is provided, the uncovered grid cells are filled with the fill_value

[BSchilperoort]

Thanks for the review @slevang !