Fix passing resolution argument to ghostscript in lattice parser

[0xteo]

The lattice parser has a resolution attribute that is never used. A default value of 300 is instead passed to ghostscript.
I changed the ghostscript command to use self.resolution as it was intended.

[codecov-io]

Codecov Report

Merging #396 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #396   +/-   ##
=======================================
  Coverage   87.69%   87.69%           
=======================================
  Files          13       13           
  Lines        1511     1511           
  Branches      349      349           
=======================================
  Hits         1325     1325           
  Misses        129      129           
  Partials       57       57

Impacted Files	Coverage Δ
camelot/parsers/lattice.py	94.11% <100%> (ø)	⬆️

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