resolves: Add outputs and descriptions

[JavierCladellas]

• Closes Insert benchmark description  #74

• Closes Add toolbox checkers #76

• Added a table showing parameters and containing links to testcase descriptions
  

• Added general description of the use case ( must be exported by the application, or found somewhere )

• Added possibility to export outputs as performance variables

• Reference values and thresholds for outputs will be supported in Add reference values and thresholds #154

Main changes:

• Added a copyParametrizedDescriptions method to feelpp/benchmarking/reframe/outputs.py OutputHandler class ( files are copied to the same directory as reports are exported right before the reframe's performance step ) And a copyDescription method that copies a single case description, once, right after reframe's init step.
• Changed item structure for generated reports (plots.json, reframe_report.json, parametrized_partials/*.adoc)
• Added the table to the template, and some methods to AtomicReport for parsing the parameters and finding the description files
• The syntax for specifying additional files is the following:

"additional_files":{
    "description_filepath":"path/to/file/my_case_description.adoc",
    "parameterized_descriptions_filepath":"{{output_directory}}/{{instance}}/{{use_case_name}}/my_parametrized_desc.adoc"
}

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