Improve rules for simplification of fractions

[gkronber]

The simplifier for symbolic expressions should try to reduce the number of parameters within fractions when possible.

(c1 x) / (c2 y) should be simplified to c3 x / y with c3 = c1 / c2

Similarly, (c1 a + c2 b) / (c3 c + c4 d) should be simplified to (c5 a + c6 b) / (c7 c + d)