User should receive a warning when the given data set can not cover the target grid.

[BSchilperoort]

I think the user should receive a warning when the given data set can not cover the target grid. So it is better to have an extra check here. Currently, if that is the case, the regridder still works, but user will get a field with only 0 outside of the area covered by the original data.

Originally posted by @geek-yang in #12 (comment)

[BSchilperoort]

Additionally, we should check if the new resolution is finer than the old one. In that case an error should be raised instructing the user to use something like nearest-neighbor

[BSchilperoort]

#31 made sure that the code won't fail. However, we should still give a warning.