lisa: PerfAnalysis: cleanup

• Move to lisa.analysis.rta

• Give "action" name to methods where appropriate ('get_' prefix)

• turn tasks() into a property

• Streamline __init__ so it takes the common denominator between all ways of building the object.

• Add alternative factories:

  • from_log_files
  • from_task_names
  • from_dir

  note: from_dir() has a similar behavior to the former constructor.

• Rename camelcase methods to PEP-8 naming style like the rest of the code

• Remove leading double undescores from method names to avoid name mangling.

• Split plot_perf into 3 separate plot methods, like it is done in other analysis classes

• Remove dead code here and there

• Use pandas.read_csv instead of read_table to avoid FutureWarning

Changes to AnalysisBase:

• Split into AnalysisHelpers and AnalysisBase
• Improve AnalysisHelpers.save_to() to infer the image format from filepath.

EDIT: updated to match latest commit message