add optional (non-default) nested symbol table

[codecov-commenter]

Codecov Report

Patch coverage: 97.38% and project coverage change: +1.61 :tada:

Comparison is base (5eaad18) 93.80% compared to head (f511b1a) 95.42%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #121 +/- ## ========================================== + Coverage 93.80% 95.42% +1.61% ========================================== Files 4 5 +1 Lines 1711 2775 +1064 ========================================== + Hits 1605 2648 +1043 - Misses 106 127 +21 ``` | [Impacted Files](https://app.codecov.io/gh/newville/asteval/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matt+Newville) | Coverage Δ | | |---|---|---| | [asteval/asteval.py](https://app.codecov.io/gh/newville/asteval/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matt+Newville#diff-YXN0ZXZhbC9hc3RldmFsLnB5) | `90.69% <84.00%> (+0.13%)` | :arrow_up: | | [asteval/astutils.py](https://app.codecov.io/gh/newville/asteval/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matt+Newville#diff-YXN0ZXZhbC9hc3R1dGlscy5weQ==) | `92.69% <93.82%> (+3.35%)` | :arrow_up: | | [tests/test\_asteval.py](https://app.codecov.io/gh/newville/asteval/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matt+Newville#diff-dGVzdHMvdGVzdF9hc3RldmFsLnB5) | `97.45% <97.70%> (-0.33%)` | :arrow_down: | | [tests/test\_asteval\_orig.py](https://app.codecov.io/gh/newville/asteval/pull/121?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Matt+Newville#diff-dGVzdHMvdGVzdF9hc3RldmFsX29yaWcucHk=) | `97.78% <97.78%> (ø)` | |

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