Fix issues with traits file format

* README.md: update header: Traits ==> Trait
* gn3/computations/qtlreaper.py: update header: Traits ==> Trait
* qtlfilesexport.py: Choose only BXD strains

  Rename the first column header from "Traits" to "Trait" to correspond with
  what `rust-qtlreaper` expects.

  Choose only the BXD strains for the proof-of-concept example - this helped
  bring out the fact that the traits file SHOULD NOT contain a strain column
  for a strain that does not exist in the genotype file in consideration.

  If the traits file has a strain column which does not exist in the genotype
  file, then `rust-qtlreaper` fails with a panic, since, from what I can tell,
  it tries to get a value from the genotype file for the non-existent strain,
  which results to a `None` type. Subsequent attempts at running an operation
  on the `None` type lead to the panic.

0 files changed, 0 insertions, 0 deletions