Refactor: transform submodule; Field storage control; CondSRF optimization

[MuellerSeb]

Closes #195 #196

• gstools.transform:
  • add keywords field, store and process keyword to all transformations to control storage and respect normalizer
  • added apply_function transformation
  • added apply as wrapper for all transformations
  • added transform method to all Field (sub)classes as interface to transform.apply
  • added checks for normal fields to work smoothly with recently added normalizer submodule
• Field:
  • allow naming fields when generating and control storage with store keyword
  • all subclasses now have the post_process keyword (apply mean, normalizer, trend)
  • added subscription to access fields by name (Field["field"])
  • added set_pos method to set position tuple
  • allow reusing of present pos tuple
  • added pos, mesh_type, field_names, field_shape, all_fields properties
• CondSRF:
  • memory optimization by forwarding pos to underlying krige instance
  • only recalculate kriging field if the pos tuple changed (optimized ensemble generation)
• performance improvement by using np.asarray instead of np.array where possible
• updated examples to use new features

[MuellerSeb]

@LSchueler this is ready for review.

[LSchueler]

Wow! Awesome!
Just a few minor comments in the code, as usual.

And I fixed a typo and some forgotten "optional" descriptors for the now optional pos argument. So, don't forget to pull.