Make mcp evolution algorithm a bit more readable.

In particular, distinguish between cost and cumcost, the same
variable name was used for this.
Improve speed of MPC by about 30%

Dear diary, I spend a whole day searching for why this implementation
of the MPC was slower then my own implementation. I checked *everything*.
I rewrote the front evolution to match with mine almost exactly. I was
about to give up. And then I saw this... Arg!
MCP: turn while loop into for loop to prevent infinite looping.

A nice feature during development :)
MCP: add sampling attribute: deal with anisotropic data.
MCP: refactored to use typed memoryviews

The arrays are stored on the mcp object, so that they can
be used in methods as well. Will use that to make MCP more
flexibel.
update-mcp: Refactor MCP algorithm in prep for MCP subclasses

Installed a few hooks to allow more flexibility in subclasses, refactored
traceback() to allow obtaining tracebacks fast during front evolution, and
moved some initializations to reset().

Full list:
* integer division in calculating indices
* traceback_offsets use -2 for uninitialized, -1 for seed point. In that
  we we can distinguish between these two cases in traceback()
* Clearing of traceback_offset, cumulative_costs, pushing start positions
  into the heap, are now all done in reset()
* Reset is called every time that find_cost is called.
* Add hook _goal_reached()
* Add hook _examine_neighbor()
* Add hook _update_node()
* Replace while loop with a for-loop to prevent infinite looping in case of a bug.
* Split traceback() in two functions to allow getting a flat traceback fast.
  traceback() calles these two private functions and behaves as just before.
update-mcp: Add MCP_Connect class

This class finds the connections between the given seed points. This is
a bit similar to finding connections between start and end points, except
now start- and endpoints are the same.
update-mcp: Add MCP_Flexible subclass.

This class adds a public def-method for the private cdef-method hooks.
This allows users to influence the behavior of the MCP algorithm by
subclassing it in pure Python.