lots of javadocs, lots of cleanup, better float test, tho not complete

I have removed all public no-arg constructors from Ptgs .. to me its a 
nightmare scenario where api users create blank ptgs and write them to disk. 
All constructors being called from the parser have been made protected. only 
public contructor in ptg is the one with byte array. 

I have left Area3dPtg alone since NameRecord depends on it, and AttrPtg coz i 
dont understand it yet. 

I have deleted FormulaUtil, since it cant be compiled any more. model/Sheet has 
minor change, FormulaViewer has changes to compile, but i havent checked to 
make it work.