atom=X option broken

Setting the `atom=` option causes fast_ep to fail:
```
$ fast_ep data=/dls/i04/data/2021/cm28182-3/processed/20210716/test_crystal_1/fast_dp/fast_dp.mtz machines=10 atom=S
Using 4 cpus / 10 machines
Input: /dls/i04/data/2021/cm28182-3/processed/20210716/test_crystal_1/fast_dp/fast_dp.mtz
N try: 200
Dataset: sad
Columns: I(+),SIGI(+),I(-),SIGI(-),merged
Unit cell: 58.20 58.20 150.54 90.00 90.00 90.00
Pointgroup: P 4 2 2
Resolution: 29.10 - 1.78
Nrefl: 47611 / 21724
DF/F: 0.037
dI/sig(dI): 0.935
Spacegroups: P422 P4212 P4122 P41212 P4222 P42212
SHELXC summary:
Dmin %comp
9.54 131.70 82.30 2.64
5.87 106.10 96.50 2.06
4.42 104.60 99.60 1.43
3.61 100.10 100.00 1.19
3.09 76.90 100.00 1.12
2.72 48.50 100.00 1.00
2.44 30.20 100.00 0.86
2.22 22.90 100.00 0.83
2.05 16.40 100.00 0.80
1.90 9.90 100.00 0.78
1.78 5.40 98.90 0.76
Anomalous limits: 1.78
Running 30 x shelxd_mp jobs
*** FIND: All SHELXD tasks failed to complete ***
```