doc and test VGCCParams

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.89 :warning:

Comparison is base (42037b5) 36.92% compared to head (5e50862) 36.03%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #226 +/- ## ========================================== - Coverage 36.92% 36.03% -0.89% ========================================== Files 54 63 +9 Lines 11150 11497 +347 ========================================== + Hits 4117 4143 +26 - Misses 6792 7113 +321 Partials 241 241 ``` | [Impacted Files](https://app.codecov.io/gh/emer/axon/pull/226?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer) | Coverage Δ | | |---|---|---| | [chans/vgcc.go](https://app.codecov.io/gh/emer/axon/pull/226?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer#diff-Y2hhbnMvdmdjYy5nbw==) | `60.00% <100.00%> (ø)` | | ... and [8 files with indirect coverage changes](https://app.codecov.io/gh/emer/axon/pull/226/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=emer)

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