Create utils-columns.R

[ebenp]

Added A function to get the column number from an Upper or lower case letter.

• [x] checked against latest main branch

• [x] documented following package documentation

• [x] checked lower and upper case cases

[ebenp]

Thanks. I added some warning code. I think this is a good addition.

[ebenp]

@jzadra Updated for 2 and 3 letter combinations, error checking and to allow vectors. Tests I ran included:

letnum("A")
letnum("Aa")
letnum("Aaz")
letnum(1)
letnum("1")
letnum(c("AA", 1, 23))