Incorrect vector id-substitution by $-variables for dot products

[tueda]

Probably related to #607, but it may be a different bug. This is the opposite replacement of #568 (id $k = newvector), where the roles of LHS and RHS are swapped.

The result is incorrect when replacing a vector contained in a dot product with another vector using a $-variable in the Identify statement. For example,

V p,q,k;
L F = p.q;
#$q = k;
id q = $q;
P;
.end

gives

   F =
      k;

A workaround is to use replace_:

multiply replace_(q,$q);

which gives the expected result:

   F =
      p.k;