GET DIRECT DATA on a subdimension generates zero size lookups

[ToddFincannon]

When an array on a subdimension reads data from a CSV file using GET DIRECT DATA:

DimM: M1, M2, M3 ~~|
SubM: M2, M3 ~~|

q[SubM] = GET DIRECT DATA('e_data.csv', ',', 'A', 'B2') ~~|

The generated lookup size is zero:

_q[2] = __new_lookup(0, /*copy=*/true, (double[]){  });

This causes a memory read error (see #161).

[ToddFincannon]

We were using the index in the subdimension family instead of the subdimension itself. The family size is larger than the subdimension size, so the CSV read would come up empty, resulting in a zero size lookup. The solution is to detect which situation is in effect and use the correct index.