Update database_schema.rst

dbsnp updated to version 137
Merge pull request #203 from tanglingfung/patch-3

Update database_schema.rst, thanks to @tanglingfung
Extended PED format added.

The PED file loaded with GEMINI can now have arbitrary columns after the
requisite 6, if a header starting with # is added. The columns in the GEMINI
database will be the names in the header.

For example:
\#family_id sample_id paternal_id maternal_id sex phenotype ethnicity hair_color
1 M10475    None None  1    1    None   brown
1 M10478     M10475  M10500    2    2    None   brown
1 M10500     None    None    2    2    None purple
1 M128215    M10475  M10500    1    1    None blue
Merge branch 'master' of github.com:arq5x/gemini
Clean up some unused options for gemini query.
Fixed variant_samples etc all showing up when not requested.
Clarity edits, some docstrings added.

Removed unused partition function.
[ENH] auto_* tools work without parents
[DOC] update auto_* tools re: parents no longer required.
tweaks to auto_rec functionality for unaffected children
resolve conflicts
add missing test files
fix merge goof
Add a --devel flag to update allowing explicit retrieval of latest GitHub version
add VEP format for version 73
Merge pull request #222 from udp3f/master

add VEP format for version 73
bump to v0.6.2
fix docs