crosscorr: bug fixes in pairwise_covariances

propagate: improve speed of gpu propagation by about 10% by coalescing some kernels. Write some new functions for doing frame-wise normalization (rather than normalizing the whole 3d array) on the gpu. This improves the responsiveness of the imaging_backend.

symmetry_microscope: include option to even run the despising.

gpu: modify k_exec to deal with local memory

conditioning: bug fixes

kernels: new median filters which are faster by amount 2x (particularly median filter 3).
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Changes to the speckle server to support panning and zooming of data displays through use of the svg transform attribute, in particular scale() and translate(). Implementing this change requires numerous changes in the front end and some changes in the backend. In the backend, there is no longer a need for making as many images, as zooming is handled with a single image rather than a series of sprites