PANDA- adding guard against invalid gene IDs and test case.

[blawney]

To address https://github.com/netZoo/netZooPy/issues/341

Note that the original issue is only raised when the modeProcess arg to the Panda constructor is either legacy or intersection.

For the default of modeProcess='union', there is apparently no problem (i.e. no errors/exceptions) even if the expression matrix genes do not have any intersection with the motif priors. Unsure if that's an issue, or whether that just triggers a trivial run (i.e. there is no corresponding gene expression data so the iterative looping performs no updates)? If so, maybe a warning should be generated?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.90%. Comparing base (ac54083) to head (2b5dc6a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #343 +/- ## ========================================== + Coverage 58.79% 58.90% +0.11% ========================================== Files 39 39 Lines 2541 2543 +2 ========================================== + Hits 1494 1498 +4 + Misses 1047 1045 -2 ```

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