Make sure initial context is Numeric

[dpvc]

This PR addresses Alex's comments concerning the default MathObject context when no Context() is specified. It does so by explicitly setting the context to Numeric, which is what was supposed to have happened anyway, but do to a timing problem, that wasn't the case; the PG environment values weren't being added to the context properly. So tolerance, useBaseTenLog and some other values were not the same as they would be if Context("Numeric") was performed explicitly.

This can be tested by using:

DOCUMENT();
loadMacros(
  "PGstandard.pl",
  "MathObjects.pl",
);
TEXT(Context()->flag('tolerance'), $BR);
Context("Numeric");
TEXT(Context()->flag('tolerance'), $BR);
ENDDOCUMENT();

With the patch, both values should be the same, without the patch, they will be different by a factor of 10.

[mgage]

This fixes the initialization problem