Add fva_sol and essential_genes params to KBaseFBABuilder.fromCobra

fva_sol is the object returned by cobra.flux_analysis import flux_variability_analysis

essential_genes is a set containing the id's of the essential genes.

Ones essential genes is added to the builder object. A simple function could be added:

```
def gene_essential(self, gene_id):
    return gene_id in self.essential_genes
```

