1. important algorithmic improvements of XSSFRow and XSSFSheet, removed LinkedList...

1. important algorithmic improvements of XSSFRow and XSSFSheet, removed LinkedList in favor of TreeMap, that allowed O(Log(N)) performance instead of O(N) when adding new rows and cells2. Revised cell value accessors in XSSFCell. Now both HSSF and XSSF handle various cell types equally. The same exceptions are thrown in case of type mismatch, same behaviour when setting nulls, etc.
3. Moved FormulaError codes out of the Cell interface into enum. Interface isn't a proper place for it
4. Finally finished javadoc on XSSFRow and XSSFCell

git-svn-id: https://svn.apache.org/repos/asf/poi/branches/ooxml@707445 13f79535-47bb-0310-9956-ffa450edef68