$$ Write functions deleted row sum and deleted col sum returning vectors containing,

respectively, the deleted row and column sums of a matrix M passed as argument.

The functions should ensure that the matrix is square; if the matrix is not square, the

functions should return an error message.

$$ Write a function gershgorin disks returning a list with two data frames, one called

row disks and the other col disks. Both data frames will contain two columns: one

called centre with the coordinates of the disk centre and the other radius with the

radius of the disk. The data frame with row disks will be based on deleted row sums,

the one with column disks will be based on deleted column sums. The function should

ensure that the matrix is square; if the matrix is not square, the function should return

an error message.