Some minor cleanup to latexStrWidth.R
Metrics are now output into a seperate file

The TeX log file is no longer used to output metrics.  This has
advantages:

  - It is no longer necessary to read in the whole log file and then
    grep for specific strings.

  - This will hopefully make it easier to start one TeX instance,
    running on a pipe, per graphics device.  Metrics will then be dumped
    to output files for R to read.
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