Support for complex number calculations (-j) with Flux

For processing power system signals, some times it is required to perform DFT or FFT operations on the acquired signals. this require the support of complex number -j. Adding this feature to the flux math library would be more helpful to the community of electrical.

The issue points to the Flux math library but names no files, tests, or entry points. First locate the library and establish how numeric values and arithmetic are represented; clarify the required -j syntax and DFT/FFT scope with maintainers. Done means Flux supports the agreed complex-number calculations with coverage for the requested power-system use case.

Written by the indexing model from the issue text.