Discrete Direction Filter

[tjlaboss]

Description

A tally filter to bin current to the nearest discrete direction. Given a particle with a known direction and a filter with a known set of angles, tally the particle into the bin with the nearest angle.

Possible names: DiscreteDirectionFilter or DiscreteOrdinateFilter

Application: producing neutron current boundary sources for $S_N$ transport.

Alternatives

• Use a PolarFilter and an AzimuthalFilter with bins manually set midway between points on a product-set quadrature
• Instead of binning, tally a Legendre Expansion
• Go all the way and implement filters for common $S_N$ quadratures (possibly related to @nelsonag's idea in #136 ?)

Compatibility

Addition of new feature with no change to existing features.