Add exact frequency-domain translation of discrete systems to continuous time

[codecov[bot]]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (ee66b95) 33.86% compared to head (5627ee3) 33.85%.

Files	Patch %	Lines
lib/ControlSystemsBase/src/discrete.jl	0.00%	13 Missing :warning:
lib/ControlSystemsBase/src/types/DelayLtiSystem.jl	0.00%	3 Missing :warning:
lib/ControlSystemsBase/src/types/LFTSystem.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #902 +/- ## ========================================== - Coverage 33.86% 33.85% -0.01% ========================================== Files 42 42 Lines 4651 4667 +16 ========================================== + Hits 1575 1580 +5 - Misses 3076 3087 +11 ```

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[JuliaControlBot]

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