Create as.vpts() function

[peterdesmet]

It is currently possible to convert a vpts object to a dataframe with as.data.frame(). The reverse is not possible, i.e. when you have a dataframe with vpts data, you cannot create a vpts object (to plot or use for other functions).

In the short term, it would be good to have such a function create_vpts(df). This function could then also be used by the new read_vpts() function (#551).

In the longer term, vpts objects should become data frames (rather than currently lists), so it is easier to use e.g. dplyr::filter() and import data from vpts csv without having to convert them to objects.

[bart1]

Quickly tagging @jedgroev as he might be intrested here

[adokter]

included in #590