New public function write_results!

[ahojukka5]

write_results!(analysis, writer) is a function which is used to implement new results writers for some analysis. Usage example in pseudo-level

type XdmfWriter <: AbstractResultsWriter
    # stuff
end

type Dynamics <: AbstractAnalysis
    # stuff
end

function FEMBase.write_results!(analysis::Dynamics, writer::XdmfWriter)
    # fid = open(writer.filename)
    # for problem in get_problems(analysis)
    #     field = problem("displacement", analysis.time)
    #     write(fid, field)
    # end
    return nothing!
end

[coveralls]

Coverage remained the same at 100.0% when pulling b85dbd3 on results_writer_abstraction into 2dba599 on master.