Replace small values by 0: epsilon2zero

### 2.54.1 Replace small values by 0: `epsilon2zero`

`epsilon2zero` takes as argument an expression of `x`.
`epsilon2zero` returns the expression where the values of modulus less than `epsilon` are replaced by zero. The expression is not evaluated.
The `epsilon` value is defined in the `cas` configuration (by default `epsilon=1e-10`).
Input :

`epsilon2zero(1e-13+x)`

Output (with `epsilon=1e-10`) :

`0+x`

Input :

`epsilon2zero((1e-13+x)*100000)`

Output (with `epsilon=1e-10`) :

`(0+x)*100000`

Input :

`epsilon2zero(0.001+x)`

Output (with `epsilon=0.0001`) :

`0.001+x`