Add basic tests for functionality

[mbercx]

E.g. plotting: does

gnuplot -p -e 'plot sin(x)'

work? Think about what we can test in the CI and add it.

[mbercx]

For xcrysden:

test.sh

xcrysden --xsf Si.xsf --script <(echo 'scripting::printToFile /tmp/si.png windowdump; exit') && echo OK || echo FAIL

Si.xsf

CRYSTAL
PRIMVEC
  5.43  0.00  0.00
  0.00  5.43  0.00
  0.00  0.00  5.43
PRIMCOORD
2 1
14  0.000  0.000  0.000
14  1.358  1.358  1.358