Use table column names as variables in eval and add tests

[pjotrp]

bio-table can use column names as variable in eval strings, which makes for sweet syntax using Ruby meta-programming. Example, if a column name is FDR

bio-table table1.csv --num-filter "fdr <= 0.05" 

works. What is needed is normalisation of column names, checking for duplicates and tests for all scenarios. In all an implementation is needed that needs to be more robust than the current version.