Fix bilinear interpolation over uniform grid
Allow separate incident/exitant CollectorSpheres

Allow TransferMatrixJob to use a separate CollectorSphere for incident
directions and for exitant directions (if different CollectorSpheres are
used, the resulting matrices would not necessarily be square).
Allow adjoint tracing for TransferMatrixJob
Strategies for reparameterizing exitant vector

Add ExitantVectorStrategy to allow for reparameterization of the exitant
vector for a TransferMatrixJob.
Builder for creating TransferMatrixJob instances
Fix error when fixed-size block has wrong length
Add methods to write cell-arrays of strings

Add methods to MatlabWriter/MatlabOutputStream to support writing
cell-arrays containing strings.
Add PDF that generates a fixed value (Dirac PDF)
Add support for variable-wavelength channels

Allow channels in a TransferMatrixJob to consist of a distribution over
the wavelength domain, rather than being restricted to a fixed
wavelength.