Calculates values of a selection index given trait values and
weights. This function is intended to be used in combination with
selection functions working on populations such as
`selectInd`

.

## Examples

```
#Create founder haplotypes
founderPop = quickHaplo(nInd=10, nChr=1, segSites=10)
#Set simulation parameters
SP = SimParam$new(founderPop)
SP$nThreads = 1L
#Model two genetically correlated traits
G = 1.5*diag(2)-0.5 #Genetic correlation matrix
SP$addTraitA(10, mean=c(0,0), var=c(1,1), corA=G)
SP$setVarE(h2=c(0.5,0.5))
#Create population
pop = newPop(founderPop, simParam=SP)
#Calculate Smith-Hazel weights
econWt = c(1, 1)
b = smithHazel(econWt, varG(pop), varP(pop))
#Selection 2 best individuals using Smith-Hazel index
#selIndex is used as a trait
pop2 = selectInd(pop, nInd=2, trait=selIndex,
simParam=SP, b=b)
```