Fix semisimple -> simple in symmetry reduction

[blegat]

First step of https://github.com/jump-dev/SumOfSquares.jl/issues/302

Closes https://github.com/jump-dev/SumOfSquares.jl/pull/307 Closes https://github.com/jump-dev/SumOfSquares.jl/pull/306 Closes https://github.com/jump-dev/SumOfSquares.jl/pull/305 Closes https://github.com/jump-dev/SumOfSquares.jl/pull/304 Closes https://github.com/jump-dev/SumOfSquares.jl/pull/303

[codecov[bot]]

Codecov Report

Patch coverage: 88.50% and project coverage change: +3.77 :tada:

Comparison is base (8476c96) 80.11% compared to head (15066a1) 83.89%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #308 +/- ## ========================================== + Coverage 80.11% 83.89% +3.77% ========================================== Files 41 41 Lines 2112 2179 +67 ========================================== + Hits 1692 1828 +136 + Misses 420 351 -69 ``` | [Impacted Files](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/308?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev) | Coverage Δ | | |---|---|---| | [src/Certificate/Symmetry/block\_diag.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/308?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL0NlcnRpZmljYXRlL1N5bW1ldHJ5L2Jsb2NrX2RpYWcuamw=) | `90.72% <88.50%> (+90.72%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/308/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.