faster timeexpand

[behinger]

• faster time_expand by more type stability
• restructured LMM tests
• started to make Basis.kernel obsolete, more type stability in time_expand, better modularity

[behinger]

Release notes:

• added rePCA for LMMs
• bugfixes
• more type-stability
• potentially breaking: starting to deprecate FIRbasis.kernel field. kernel(firbasis) does not return the kernelfunction anylonger, which was required to be an anonymous function. Rather, it is replaced by kernel(firbasis,e) to directly calculate the kernel at e. Unfortunately, the change as currently implemented would break once more loading of Unfold models from older versions, as the FIRbasis Type changed and is thus incompatible. We could keep the FIRBasis.kernel field around, and just fill it with nothing - but at some point it should go..