Add options for additional image resampling methods in ImageMapper

[aribrill]

To work with standard convolutions, hexagonal camera images must be resampled into a square grid. Currently, oversampling is implemented, but other methods have been found to give better performance. A study of different methods can be found here. In rough order of priority, methods to add could include:

• Rebinning (good performance, fast)
• Nearest interpolation (? performance, fast)
• Linear interpolation (good performance, fast)
• Cubic interpolation (best performance, slow)
• Smoothed sampling, e.g. Gaussian sampling (worse performance, fast)

[aribrill]

@TjarkMiener Since there's no reason to expect that Gaussian sampling would give improved performance relative to the other methods I think it's reasonable to not implement it. Do you agree? If so we can mark this issue as resolved by #72.