LetterInventory

This implements a class called LetterInventory that can be used to keep track of an inventory of letters of the alphabet. The constructor for the class takes a String and computes how many of each letter are in the String. This is the information the object keeps track of (how many a’s, how many b’s, etc). It ignores the case of the letters and ignores anything that is not an alphabetic character (e.g., it ignores punctuation characters, digits and anything else that is not a letter).