Change the syntax for values of deterministic shocks:

Arbirtrary expressions after the "values" keywords must now be enclosed
within parentheses; consider the following example:
<code>
periods 1:2;
values -1 -2;
</code>

In the previous syntax, this was interpreted by the preprocessor as a shock of
value -1-2 = -3 for periods 1 and 2, which is clearly not the intent of the
user; with the new syntax, this will be rejected (too many values compared to
the number of ranges).

Also note that now commas are no longer required between arbitrary expressions,
since the parentheses are sufficient for separating them.