Improve flexibility of Genotype Matrix and Flat-file formats

[carolyncaron]

The current suggested formats are too restrictive.

Suggested Flat-file format:

1. Variant name (required)
2. Marker name (optional)
3. Chromosome (required, unless select only)
4. Position (required, unless select only)
5. Sample name (required)
6. Genotype Call (required)
7. Meta-data (optional) (refer to issue Assign quality metrics to each genotype as they are loaded #5 for suggested format)

Suggested Matrix format:

1. Variant name (required)
2. Marker name (optional)
3. Chromosome (required, unless select only)
4. Position (required, unless select only)
5. Genotype call (required) PLUS meta-data* (optional)

*Also see issue #5 for more context regarding how we propose to handle meta-data

Not sure whether this should be in its own issue, but it has also been discussed that users can suggest a custom format for marker name through the use of tokens with chromosome + position.