Add uncertainty input option to constructor

[btmy87]

Add option to UncVal constructor for user to input the uncertainty input type: 1-sigma, 2-sigma, 95%, uniform, ...

• The constructor will handle the appropriate calculation to get the variance.
• PDF and CDF functions could do something different for uniform.

• 6 could generate different samples for uniform distribution.

[btmy87]

Implemented in 34255e85660f830bd59650af6015ca86e577b228