expand visits into rows

option/processing function to take e.g.

eid, 1-0.0, 1-1.0
1,   a1,     a2
2,   b1,     b2
3,   c1,     c2

and transform it into this:

eid, visit, 1.0
1,   0,     a1
1,   1,     a2
2,   0,     b1
2,   1,     b2
3,   0,     c1
3,   1,     c2