[trans] Integer output from integer data

The data in FIPNUM1 and FIPNUM2 in the outputted csv below is possible to write in integers:

```
$ ecl2csv trans EIGHTCELLS.DATA --boundaryfilter --vectors FIPNUM -o -
I1,J1,K1,TRAN,I2,J2,K2,DIR,FIPNUM1,FIPNUM2
1,1,1,42.63500213623047,1,1,2,K,1.0,2.0
1,2,1,42.63500213623047,1,2,2,K,1.0,2.0
2,1,1,42.63500213623047,2,1,2,K,1.0,2.0
2,2,1,42.63500213623047,2,2,2,K,1.0,2.0
```

(this is on a modified EIGHTCELLS deck with lower layer set to FIPNUM 2)

Reproduce the reported output with `ecl2csv trans EIGHTCELLS.DATA --boundaryfilter --vectors FIPNUM -o -`, using the modified deck described in the issue. Trace the `trans` CSV output path and inspect how FIPNUM1 and FIPNUM2 values are formatted. Done means integer-valued FIPNUM columns are emitted as integers rather than `1.0` and `2.0`.

Written by the indexing model from the issue text.