Got started on these two tasks, restructuring still needs a lot of work here

Code updated for static training, tests not done yet

New class 'Engine' responsible for both training sequentially and non-sequentially.
Currently training with old MetaModEngine and in MetaMod directly still possible, but will be deprecated and removed soon.
Also started splitting ExpDesign into InputSpace (for MetaMod, created dists and polynomials) and ExpDesign (for Engine, can also sample)

Started testing MetaModel while also refactoring a bit more by moving code between ExpDesign and MetaModel

Added tests for Inputs and started on those for ExpDesign.
Regarding the second there are some open questions noted in issue #32

[postprocessing] remove save_fig arguments and PdfPages.

See merge request !7

[bugfix] Fix bug for save_fig=False. Do not create a temporary outputfile.

See merge request !4

Resolve "Justifiability analysis"

Closes #14

See merge request !1