Rolls over existing PerformancePeriod.

Instead of doing the rollover by creating a new PerformancePeriod,
introduces a `rollover` method that resets the values that need
to be fresh in a new period, and moves the ending values to starting
values, and leaves positions intact.

This isn't a major runtime improvement in of itself, but it does
allow us to more easily keep track of position values from period
to period, which other improvements will use.
Uses vdot and numpy arrays for position totals.

Gets almost 100x speed up over iterating over the values and
summing up the values in Python.

Farms out the work to numpy and atlas by using the vector dot
product of the amounts and last sale prices.

Adds some wiring of keeping track of an index into the numpy arrays
for each position, so that value can be overwritten as events update
those amounts and sale prices.