Complex Number Support 2

[xtalax]

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the ScioML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Adds complex number support, does this by splitting the system in to real and imaginary parts.

fixes https://github.com/SciML/MethodOfLines.jl/issues/58 fixes https://github.com/SciML/MethodOfLines.jl/issues/101

depends on https://github.com/SciML/PDEBase.jl/pull/22

[codecov[bot]]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (a04e094) 83.02% compared to head (e240a06) 84.79%. Report is 40 commits behind head on master.

Files	Patch %	Lines
src/interface/solution/common.jl	21.42%	11 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #342 +/- ## ========================================== + Coverage 83.02% 84.79% +1.76% ========================================== Files 40 41 +1 Lines 1756 1960 +204 ========================================== + Hits 1458 1662 +204 Misses 298 298 ```

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