Allow for different time-windows per event

[behinger]

To implement this the following things have to change:

• specification of time-window in uf_timeexpanddesignmat and uf_epoch
• Fill the "missing" betas with 0's at the point of reshaping
• check the plotting functions
• check interaction with basis functions

It could very well be that some things are not supported anymore. E.g. time-basis functions.