MAINT: Move risk to its own module.

In anticipation of splitting apart the different risk classes
into their own submodules, a distinct risk module should help
organize those new classes.
MAINT: Split apart risk metrics classes.

Also remove test that compares risk metrics batch to iterative,
since the 'iterative' calculations, replaced by the cumulative
calculations, will intentionally drift from the results in the risk
report due to annualization and other factors.

Work towards having separate calculations for the fixed periods versus
the cumulative/headline risk metrics.
Different sumbodules for each type should help make the calculations
type distinct and easier to find.
MAINT: Break period and cumulative risk metrics into submodules.

In anticipation of changing the sharpe, beta, et al. calculations
dependent on whether the period returns or the overall returns
are being calculated.
Merge beginning of divergence of the risk calculation types.