#7 Path variables in config file

[John Foerch]

File paths in the config file should be able to include
variables like JJFM_BASEDIR, JJFM_GEODIR, and other
convenient directories. This would make scripts more
portable because they wouldn't rely on relative
pathnames with "..". I suggest that these variables be
indicated by a leading dollar sign like $JJFM_GEODIR.

[John Foerch]

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I have implemented the basic mechanism for this, along with
a couple of variables. Namely ${geodir} and ${basedir}.
Each geolayer has a function that is responsible for parsing
its config tag. This function now receives a pointer to the
symbol table. This way, only attributes deemed as "safe"
can use symbolic variables.

At the moment, variables that contain paths also contain a
trailing / character. However, after using this capability,
it does not look right to me. I may change it before the
official release of version 14.

[John Foerch]

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With my repository commit tonight, I have changed the syntax
of path variables so that they do not include a trailing /.