fix: Execute nested functions

[adlerfaulkner]

Fixes #30

With this change, there are now three places where the code repeats a nearly identical block to execute a function:

const definition = functionHelper.findDefinition(data, functionValue.predicateObjectMap, prefixes);
const parameters = functionHelper.findParameters(data, functionValue.predicateObjectMap, prefixes);
const calcParameters = await helper.calculateParams(Parser, parameters, index, options, data, prefixes);
const result = await functionHelper.executeFunction(definition, calcParameters, options);

When I have some time I can start refactoring that stuff, unless someone is already working in that direction.
Some other work I'd love to see & help with in this repo is:

• Conversion to typescript
• More adherence to linting, I'm seeing tons of linter errors

[ThibaultGerrier]

Hey there, really cool stuff, seems to work well!

Tried it out with some yarrrml mappings with nested functions and it performs great:) Seems like the changes necessary to support this in the end were rather simple.

Also means that yarrrml conditions (https://rml.io/yarrrml/spec/#conditions) now work as well. Although the extra function yarrrml adds to the mapping would need to be added manually when using that feature. Something like the following seems to work:

      'http://example.com/idlab/function/trueCondition': (args) => {
        if(args['http://example.com/idlab/function/strBoolean']) {
          return args['http://example.com/idlab/function/str'];
        }
      },

Regarding this project's code quality, yeah it's not great. Not sure how feasible a conversion to typescript would be, adding type annotations would be one thing but improving the code quality in general another alltogether. At that point a complete rewrite would probably make more sense.
Besides the occasional bugfixes this repo isn't really worked on.

I'll merge this PR and publish it with a new minor version to npm.

[adlerfaulkner]

Awesome, thanks!

I may start working on the complete rewrite soon then. Will keep you updated.