Fixes bug in DiscreteFixed

Previously a probability of 0 was always returned. Now the scheme
returns the fixed probability passed through the options.
Fixes retrieval of derived variable

When determining which dataset to pull a derived variable from, an
incorrect assertion checks that the input dataset has the derived
variable and not the base variable.
Improves calculation of ensemble min and max

getMin returned MV when any of its members were missing
Allow discrete probabilities to be 0 or 1

Previously asserts would trigger when discrete probabilities were 0 or
1.
Fixes ContinuousBpe when several ensemble members are equal
Fixes bug in Global::getLower/UpperIndex in case of ties
Fixes the checking of options to MeasureFraction

One of three options has to be specified, but the logic that determined
if this was the case was wrong.
Fixes return type of Deterministic value

Return type of Deterministic and Value objects were int when it should
be float. This caused forecasts to be rounded in the last few commits.
Fixes bug in CalibratorPitBased for variables with discrete mass points

A logical error was made where the probability mass at the upper
boundary (P1) was infact computed as 1-P1. This caused the method to
short-circuit and often return the original CDF without being
calibrated.
Fixes bug in getting inverse Cdf from Continuous for discrete variables

The iterative getCdfInv method does not converge when the probability
mass at the lower boundary is greater than the CDF sought. The correct
way is to return the x-value at the lower boundary in this case. Same
problem for the upper boundary.
ObsMetric doesn't need a forecast to be valid
Added forceLimits option to Input

Values outside a variable's range can now be rounded up to the lower
boundary or down to the upper boundary.
Fixes bug in QcDefault for variables with one discrete point
Improves InterpolatorLinear when multiple points have the same x-values.

Uses the average of the smallest and largest y-value when multiple
points have the same x-value.