Add initial condition block in "gradient.R"

Add blocks in "gradient.R" to compute the gradient with respect to
initial condition of the ODE system.

Create "gradient_test.R" in "test/".
Add coefficient gradient block in "gradient.R"

Add block in "gradient.R" to compute the gradient with respect to
coefficient matrix.

Modify "gradient_test.R" in "test/".

Remove input argument "type".
Might add back later.
Add intercept gradient block in "gradient.R"

Add block in "gradient.R" to compute gradient with respect to intercept.

Modify "gradient_test.R" in "test/".