Use class constructor methods in PolylineSet

[connortann]

I think the init method of PolylineSet is doing a fair bit of work and could be refactored. In other resqpy classes, we've tried separate out any alternative constructor options as separate class methods. An example is resqpy.well.DeviationSurvey.from_data_frame . This helps keep the init method simple.

I would suggest making class methods e.g.PolylineSet.from_irap_file() and PolylineSet.from_charisma_file()

Originally posted by @connortann in #164 (comment)

[andy-beer]

Issue #278 is a superset of this issue.