Make timestep mandatory in get_contour()

[thorbjoernl]

With the newly split contour files retrieval of all timesteps in a single requests (how get_contour worked before #106) will not work when reading the new (split) files. Re-adding this functionality for these files, would probably require low level path globing or adding a queryable index to determine which files to combine, which seems like more effort than it's worth. Instead we should make timestep mandatory.

Since this is a breaking change, this should be done as part of v0.3.0