Use options for urls as much as possible

[bart1]

In some locations urls are still hard coded, it could be good to do a round of converting these to options so cases like #118 can be resolved temporarily with ease until a new package release. This wont cover all changes as assumptions about the structure of the data are still made but at least simple fixes can be done

[bart1]

What also might good is to make the number of retries an option. For example in the dutch pvol reader