Handle A1/REF/ALT from plink2 glm

To handle plink2 glm results we may need something like this:
```
df['A2'] = [(ref if (alt==a1) else alt) for ref, alt, a1 in zip(df['REF'].values, df['ALT'].values, df['A1'].values)]
```
Sample from ``plink2 glm`` output:
```
REF ALT	A1
A	G	G	
C	T	C	
T	G	G	
C	A	C	
C	T	T	
T	C	T	
G	A	G	
G	A	A	
G	A	G	
```