Change some functions in TimeStepper to const

Functions are: order(), nprev_values() and ndt(). Should have always
been const really.

INTERFACE CHANGE: If these are overloaded anywhere in your personal code
they will need to be made const there as well.
Add resize() and default constructor for shape/dshape

* Useful to not absolutely have to initialise them immediately.

* Actually shape already had a default constructor but it didn't
  initialise everything, fixed now.