Revision 920558

Resolved multiple problems leading to inaccuracy and/or failure to compute Normal, ChiSquare and 
Poisson probabilities, Erf and Gamma functions.

JIRA: MATH-282
JIRA: MATH-301

Summary of changes:

* BrentSolver has been changed to expose its configured absolute accuracy. This solver is used by
  the default inverse cum implementation in AbstractContinuousDistribution and the hard-coded setting
  (1E-6) was limiting accuracy in inverse cumulative probability estimates. AbstractContinuousDistribution
  was changed to allow distributions to set this value and NormalDistributionImpl was changed to set it to
  1E-9 by default and allow users to configure it via a constructor argument.

* AbstractContinuousDistribution and AbstractIntegerDistribution inverseCumulativeProbability methods
  have been modified to check for NaN values returned by cumulativeProbability and throw MathExceptions
  when this happens.

* The criteria for choosing between the Lanczos series and continued fraction expansion when computing
  regularized gamma functions has been changed to (x >= a + 1). When using the series approximation
  (regularizedGammaP), divergence to infinity is checked and when this happens, 1 is returned.

* When scaling continued fractions to (try to) avoid divergence to infinity, the larger of a and b is
  used as a scale factor and the attempt to scale is repeated up to 5 times, using successive powers
  of the scale factor.

* The maximum number of iterations used in estimating cumulative probabilities for PoissonDistributionImpl
  has been decreased from Integer.MAX_VALUE to 10000000 and made configurable.