Make ITime easier to use

Some patterns are repeated a lot when using ITime. Some of these could have convenience functions. For example:

`hasdate(::ITime)`
`ITime(::DateTime)`
`Second(ITime)`

Define some convenience abstract types, like "ITimeNoDate", "ITimeWithDate"

Things that ITime cannot do at the moment.
- can't add or subtract a Dates.Period from an ITime
- `ITime(::ITime)` does not exist, but `DateTime(::DateTime) and Float64(::Float64) do`

Start by locating the ITime implementation and its existing conversion and arithmetic behavior. Review the listed convenience functions, abstract types, and Dates.Period operations, then clarify which API additions are in scope and define completion through coverage of the agreed behaviors.

Written by the indexing model from the issue text.