sex_to_int: Convert character sex codes to integer codes
Description
This function accepts the character sex codes accepted by eigenstrat and turns them into the integer codes accepted by plink.
Matching is case insensitive.
The correspondence is:
U:
0 (unknown)
M:
1 (male)
F:
2 (female)
Any other characters will also be mapped to 0 (unknown) but with a warning (U does not generate warnings).