Increase code coverage for src

[fingolfin]

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[fingolfin]

A good start would be to tackle e.g. ortho.jl coverage, by adding tests with a SimpleCharTable resp. its characters.

That actually also is the "solution" for a bunch of other files/code, i.e., no test for the functionality in "simple" tables.

Another big sources of uncovered code is input validations: would be good to also test those, by adding tests that explicitly invoke functions with invalid input (out of bounds) to make sure those errors are caught correctly.