Add "sample" type for entities

Hi,
Working ont he export from Biosamples to PXF, there are no 'sample' type of entities, only 
    disease = 0
    organism = 1
    patient = 2
    variant = 3
    genotype = 4
    paper = 5

(from the [Meta](https://github.com/phenopackets/phenopacket-python/blob/master/phenopacket/models/Meta.py#L6) file). Could a sample type be added?