agstub/subglacial-inversion: First release of subglacial parameter inversion code

This python code inverts ice-sheet altimetry data for two types of subglacial parameters: the basal vertical velocity or basal drag coefficient field. The forward model is based on a small-perturbation approximation of the Stokes equations, and the least-squares inverse problem is solved with the conjugate gradient method. The primary operations involve map-plane Fourier transforms and convolution over time, which are implemented with SciPy's FFT methods. The complete details are included in a forthcoming manuscript. The Jupyter notebooks in the "notebooks" directory reproduce the computational examples from the manuscript.