differentiate between sparse and non sparse basisfunctions

[behinger]

E.g. BOLD-basis is non-sparse, and the designmatrix will generally be non-sparse as well
but FIR-basis is sparse.

a new field in the basisfunction type could be specified. This field would then decide whether the designmatrix Xdc is sparse and whether it needs to be fitted by iterative solvers or not.

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Another problem: basisfunction.times is illdefined for some bases (e.g. BOLD).
maybe should be replaced by basisfunction.colnames. But e.g. for FIR we need to know at a later point, how many negative samples there are, to move the onset of the basis to the correct sample.

[behinger]

second problem is solved. But not yet through all instances of basisfunction.times, e.g. tutorials are still missing