Expression of a function in terms of generators of rational function field (bug fix)

[pogudingleb]

Alexey Ovchinnikov has reported the failure of an assertion during reparametrization on

ode = @ODEmodel(
    x'(t) = b * (x(t)^2 + 1) / (2 * a),
    y(t) = 2 * x(t) / (b * (1 + x(t)^2))
)

The problem was with computing the expressions of rational function wrt the tag variables, it is fixed.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 90.09%. Comparing base (08dabc1) to head (e471b8c). Report is 2 commits behind head on master.

Files	Patch %	Lines
src/util.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #308 +/- ## ========================================== + Coverage 90.04% 90.09% +0.05% ========================================== Files 28 28 Lines 3306 3313 +7 ========================================== + Hits 2977 2985 +8 + Misses 329 328 -1 ```

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