Add checks for interval type

[steffilazerte]

weather_dl() has a check that the interval is length one and is hour, day, or month.

stations_search() and weather_interp() also need these checks.

[farhanreynaldo]

Hi @steffilazerte, this is my first time contributing to an R package. I’ve already created a pull request (#169) and would love your guidance on how to proceed. I’m currently trying to figure out how to add tests for these changes.

[steffilazerte]

Hi @farhanreynaldo, see my comments on your PR #169, thanks for your contribution!