Revamped+, JOSS 2.0 submission

Further enhancements to the overhaul, including:

• collection of most methods in the smooth_finite_difference module as kerneldiff
• convex optimization improvements to make robustdiff run in linear time
• caching to avoid duplicate calls during optimization
• extension of utilities and loss function to better optimize all methods in the presence of outliers
• extension of TVR to better handle outliers
• improved test coverage and control of coveralls
• linted code
• added support for multidimensional data to several methods