Remove ambiguous behavior (breaking changes)

[zkamvar]

Things that should be changed:

1. behavior of factors
2. combining aweek objects

After thinking about it (with a bit of help from Rich FitzJohn), I think adding the ability to combine aweek objects with different start days was a mistake. The behavior of simply taking the first element and converting it was a mistake because it will not be obvious to the user what is happening. Combining aweek objects should do the following:

1. only combine aweek objects with the same week_start attribute
   i. if they don't have the same week_start attribute, an error is thrown, advising the use of change_week_start()
2. if combining a factor, it is converted to a character, no matter what. The user can use factor_aweek() (or something like that) to change it.
3. truncated aweek objects are no longer truncated.

Other than that, character and date objects are trivial