Promote parametric type in IComplex constructor.

[LauraBMo]

Working with polynomials with big coefficients I had an "ERROR: MethodError:..." for the IComplex constructor.
This PR solved the problem.
An MWE to reproduce the error:

const HCIA = HomotopyContinuation.IntervalArithmetic
HCIA.IComplex(HCIA.Interval(1, big(2)), HCIA.Interval(0,1))

Interval constructor handles the type promotion in line 30, but for IComplex is missing. This PR aims to add it.

[PBrdng]

Hi, this looks like a nice bug fix. Thank you!