New features

Add the xoa.plot.plot_flow() function [#9].
Improve xoa.coords.get_depth() so that it can compute depth from sigma coordinates or layer thinknesses [#8].
Add the xoa.dyn.flow2d() function [#7].
Add the xoa.regrid.extrap1d() function.
Add the xoa.filter.erode_coast() function which is specialized version of the xoa.filter.erode_mask() for horizontal data.
Add the xoa.coords.get_xdim(), get_ydim(), get_zdim(), get_tdim() and get_fdim() for quickly finding standard dimensions.

Bug fixes