An optional list to specify the time grid
where the baseline rate function is estimated.
The availble elements of the control list include
grid, length.out, from and to.
The time grid can be directly specified via element grid.
A dense time grid is suggested.
Element length.out represents the length of grid points.
The dafault value is 1,000.
Element from means the starting point of grid with default 0.
Element to represnts the endpoint of grid
with the right boundary knot as default.
When grid is missing, the grid will be generated
by seq (from package base)
with arguments from, to and length.out.