DDB: add formula

Change Syntax "Integer period" to "Number period". Add Constraints: cost >= 0, salvage >= 0, salvage <= cost, 1 <= period <= lifeTime, declinationFactor > 0 The formula using integer periods according to http://wiki.services.openoffice.org/wiki/Documentation/How_Tos/Calc:_DDB_function would be To calculate depreciation, DDB uses a fixed rate. When declinationFactor = 2 this is the double-declining-balance method (because it is double the straight-line rate that would depreciate the asset to zero). The rate is given by: rate = declinationFactor / lifeTime The depreciation each year is calculated as depreciation_of_year = MINIMUM( book_value_at_start_of_year * rate; book_value_at_start_of_year - salvagevalue ) Thus the asset depreciates at rate until the book value is salvagevalue. book_value_at_start_of_year_i = originalcost - sigma_i(1,year_i-1,depreciation_of_year_i) The current formula is missing the MINIMUM evaluation and uses only (book_value_at_start_of_year * rate), hence the TODO in the draft. Alternative algorithm allowing for non-integer periods: rate = declinationFactor / lifeTime if rate >= 1 then rate = 1 if period == 1 then oldValue = cost else oldValue = 0 endif else oldValue = cost * pow( 1 - rate, period - 1 ) endif newValue = cost * pow( 1 - rate, period ) if newValue < salvage then DDB = oldValue - salvage else DDB = oldValue - newValue endif if DDB < 0 then DDB = 0 endif