No fixing for MD runs

[foxtran]

Currently, is not not allowed to fix specific atoms during MD runs:

https://github.com/grimme-lab/xtb/blob/8964b4f21a6b86f14b9aa9697ea90140a01b1438/src/prog/main.F90#L1116

For example, from .trj:

674
 energy: -94.049856813891 gnorm: 0.314241912651 xtb: 6.7.1 (5145905)
C            4.70246869400000       10.28811103000000       -1.02645944800000
C            5.62575251800000       10.38729071000000       -2.23879553800000
--
674
 energy: -92.995791139908 gnorm: 0.944844465047 xtb: 6.7.1 (5145905)
C            5.06988918103170       10.19282259781385       -0.76578077554437
C            5.77021606477175       10.48314735909728       -2.14872274256429

And first atom is fixed.