Update communication backend to send/recv header + timestamped data

[BenjaminRodenberg]

In #1664 I introduced bulk send/recv of multiple substeps. The main motivation is to support multirate time stepping and higher-order interpolation. This, however, came with some technical debt, because I did not consistently update the communication backend. Instead, I decided to use the existing backend with three stages of communication:

1. send/recv number of exchanged timesteps n (1 int)
2. send/recv array of timestamps (n doubles)
3. send/recv array of actual data (n * dim * size doubles)

Note that we have to know n, before we can trigger the second and the third step.

A more appropriate approach would be the following:

1. send/recv a header with the number of exchanged timesteps n
2. send/recv the actual data + timestamps

This approach should also help us to remove the pretty confusing function BaseCouplingScheme::receiveDataForWindowEnd, because we would by default always attach a timestamp to the data and, therefore, we would not need to call getTime() inside receiveData to implicitly derive the time of the received data.