Bugfixes:
- r_cmb is now correctly set to 0 by misc.get_rbounds if no snap is
  available using the shape parameter
- dt_dt and ebalance processing functions use None as default for
  tstart instead of 0
- dt_dt (and hence ebalance) is computed with a first order scheme
  (as in StagYY) instead of a second order one

Internals:
- scipy version decreased to 0.17 (avoid installation of 0.19 on read
  the docs)
- some StagyyData attributes are put in _dicts, preventing modification
  by user