Consider warning when incidence objects are created with POSIXct dates

[TimTaylor]

• POSIXct has a second resolution which means using it to represent dates is not great for aggregation (see an expanded discussion on this in #104).
• Plot method used on POSIXct date objects creates a seemingly invisible output due to the default width of 1 that we use.

[TimTaylor]

closed by https://github.com/reconverse/incidence2/commit/dc8a31525008b57f100be44a1789cd51412c6db6