add test for zero mean function
move mean functions to GP::mean namespace
pass objects by value
add covariance function
fix doc
add stupid test
remove Functor<double> specialization, fix mean func
differentiate case when matrix is reduced to a vector
add zero-argument test
add nargs to Functor, and check for that
fix constructor
add assertions on construction
fix constness issues
create test for new bug
make zero() function univariate like all mean functions
symmetries matrix by hand, return plain matrix

double * selfadjointView is not supported by Eigen 3.2.0