Refactor internals to work with log time intervals

[dbrakenhoff]

Currently TTim works by computing solutions for all log time intervals, which is nice when you want to compute the heads at some point for a period of time, but is inefficient for everything else (i.e. contouring heads at a specific time).

This issue is for keeping track of changes that need to be done to the code to change TTims default behavior to compute solutions per log time interval.

• convert all internal arrays with npval dimension to a dictionary with log time intervals t_int as keys and modify array dim npval to nppar
  • in TtimModel
  • in AquiferData
  • in Element and all subclasses
• Add solve_interval()
• Add initialize_interval() to Model, Aquifer, Element and all subclasses. One challenge here is to avoid running duplicate code with inheritance WellBase and Well both have intialize() and initialize_interval()
• Add t_int to all Equations.
• check out whether lababs is computed correctly
• figure out how to efficiently compute head/potential/etc. for multiple times.

Start by showing working example for a single Well, so we can discuss/improve implementation. See PR #74 for first implementation.

[dbrakenhoff]

Won't implement. Closing this issue. See #74 for details.