Perform nnfit cleanup

Created by: scarrazza

Port the changes from #220 and #193 to master, in particular:

• memory leak fixes
• export pdf algorithm
• keep https://github.com/NNPDF/nnpdf/pull/220/commits/bbf68a30ae243766682fd1ffe9b1ea6d3a9ceffe and https://github.com/NNPDF/nnpdf/pull/220/commits/0997d701e2512c2b631b08a5c537f385be925999