Add EOMONTH (end of month) calculation function

EOMONTH() is a common Excel function because in many cases, users (all Oil & Gas industry) need to compute information based off the end of the month.

Currently a typical user will not know how to compute the End of the Month using formulas because it is not trivial (like using an out of the box function; like in excel) and therefore google for help. Most suggested solutions on Google are not accurate because they do not work for dates in December. I have yet to find one that is accurate and works for dates in December but my formula that also works for December is also very large and scares users that simply use a function in Excel.