The buffer is a single-input, single-output digital buffer which produces as
output a time-delayed copy of its input. The delays associated with an
output rise and those associated with an output fall may be different. The
model also posts an input load value (in farads) based on the parameter
input_load. The output of this model does NOT, however, respond to the total
loading it sees on its output; it will always drive the output strongly with
the specified delays.