This function is called by fit.ssm when it needs to construct
the matrix of exponent vectors stored in the basis slot.
degl(d, N, exclude = list())A number. This determines the number of columns in the output matrix
A number. This determines the number of rows in the output matrix.
(optional) A list of integer vectors. If this argument is provided, the generated matrix will not contain any rows which are only non-zero in the columns specified by any of the vectors in the list.
The algorithm works by repeated calls to comb to generate all
possible exponent vectors of a given degree until N vectors have been
generated. Any generated vector is checked to make sure that it's non-zero
entries do not match a vector provided in exclude before being added
to the output matrix.