Setup a new summary section for each timestep

Although this will lead to parsing the same input data over and over
again (setting up the smspecs for the wells), the summary files contain
this redundant information because in Eclipse, wells can appear and
disappear during the run.
Let timestep and well report be owned by summary

Instead of calling the destroy functions for the timestep and well
report objects, the ERT memory allocated for these are contained in
the summary handle and freed together with that.

The timestep and well report objects thus becomes only views into this
memory; it is no longer allowed for these to outlive the summary section
(not that this was ever sensible in the first place).
Use forward list since we don't need random access
Add virtual destructor to avoid memory loss upon free