Simplification of Waveform class

[BenjaminRodenberg]

To implement #1171 the following changes to the Waveform class are useful:

• Remove feature to store and use data from past windows. There is currently no clear use-case and it makes the management of data more complicated.
• Allow to store several data values inside of a window. Always keep the value at the beginning of the window, when a new iteration begins and automatically delete all others.

These changes should not break any existing code, but will help us to implement higher-order interpolation when subcycling is used and multiple data values are available in a window.