Provide the possibility to split data into activity-based equally-sized windows

[bockthom]

Currently, we have the possibility to split networks activity-based by either specifying the number of edges per network or by specifying the number of windows.

However, we do not have this possibility for data-based splitting, we can only specify the number of commits resp. e-mails, but not the number of windows.

So, I suggest to implement a function that computes the activity amount based on the number of wanted windows. Example:

get.size.of.equally.sized.windows <- function(input.size, number.windows.wanted) {
  size <- ceiling(input.size / number.windows)
}

In the case of activity-based network splitting, input.size is the overall number of edges.
In the case of activity-based data splitting, input.size is the overall number of commits resp. e-mails.

So, both functions split.data.activity.based and split.network.activity.based should provide a parameter number.windows and both call the above defined function get.size.of.equally.sized.windows when the parameter number.windows is given.

[In addition, one could think of providing a function for determining equally sized windows also for time-based splitting. In that case, there is no difference between network-based or data-based time-based splitting -- we only need the very first and very last date in the data source to determine a time-period for equally-sized windows given by the amount of windows wanted. However, this will only make sense after #38 is closed.]

[clhunsen]

[In addition, one could think of providing a function for determining equally sized windows also for time-based splitting. In that case, there is no difference between network-based or data-based time-based splitting -- we only need the very first and very last date in the data source to determine a time-period for equally-sized windows given by the amount of windows wanted. However, this will only make sense after #38 is closed.]

We just need this with the argument length.out instead of by: https://github.com/se-passau/codeface-extraction-r/blob/v2.2/util-split.R#L539.

[clhunsen]

[In addition, one could think of providing a function for determining equally sized windows also for time-based splitting. In that case, there is no difference between network-based or data-based time-based splitting -- we only need the very first and very last date in the data source to determine a time-period for equally-sized windows given by the amount of windows wanted. However, this will only make sense after #38 is closed.]

We just need this with the argument length.out instead of by: https://github.com/se-passau/codeface-extraction-r/blob/v2.2/util-split.R#L539.

With the changes in PR #96, the section is now here: https://github.com/clhunsen/codeface-extraction-r/blob/d421807df819ca2173421401dc9a198954b68774/util-split.R#L773

Also, the specifiy change needs to take place here: https://github.com/clhunsen/codeface-extraction-r/blob/d421807df819ca2173421401dc9a198954b68774/util-misc.R#L174
Just compute a lubridate::period from start and end date, then divide it by the number of windows (which becomes another parameter).