[F] Allow string column index for getCell method

[JohnyL]

🚀 Feature Proposal

A clear and concise description of what the feature is.

Motivation

First of all, the most used cells reference style is A1 notation. R1C1 notation is occasionally useful, but used very seldom.
Making user calculate the position of column is not the best experience. It's much simpler to use string column index for column. Moreover, people, which will read someone's code, will have to search the letter of the column.

Example

Instead of this:

sheet.getCell(1, 39).value = 123;

it's much clearer and understandable this:

sheet.getCell(1, "AM").value = 123;