In$-$class exercise: forward substitution

Write a Python file that has a function that implements forward substitution. Please follow the requirements below for full credit:

Name the file

linsolve.py$.$

The first line of the function definition should be def forward$\_$sub$($lu$\_$fac, b$)$ :

where lu$\_$fac is a $2d$ NumPy array and $b$ is a $1d$ NumPy array.

lu$\_$fac can be assumed to be the $LU$ decomposition of some matrix, with $L$ in the lower part $($assume $L_{ii}=1).$

forward$\_$sub should return y$,$ a $1$d NumPy array, that holds the solution to $Ly=b.$

Include a docstring in the function and use $-=.$

$1$