added error_floor capability to ws3dinv in writing data file and made it easier to rewrite the data file. Also, added WSInputError class for error handling.
added Example folder on top level, directory structure is preliminary

Signed-off-by: LK <LK@uni>
Merge branch 'master' of github.com:geophysics/mtpy
added plotting functions to winglink
fixed bugs in manipulating mesh in ws3dinv and rewriting the initial model file.
Merge branch 'master' of https://github.com/geophysics/mtpy
Merge branch 'master' of https://github.com/geophysics/mtpy into new_occam