Updated Divisor.m2

changes 0.1m

added getPrimeDiviors
renamed divAmbientRing to getAmbientRing for consistency
substantial speed improvement for moduleToDivisor in the graded case via a change of module2Ideal (adding an IsGraded option)
fixed bug in moduleToDivisor which would sometimes provide the wrong shift
added mapToProjectiveSpace 
made divisorToIdeal work slightly better for anti-effective divisors
added a second algorithm to divPullBack which works for Cartier divisors even in the map is not flat or finite
added getLinearDiophantineSolution which makes findElementOfDegree work in the multigraded setting
added a test which should cause the error in minimalPrimes when applied to the unit ideal to be obvious