Integration Fixes

[tylerflex]

Things that dont work properly with convert that need to be fixed with integration.

• plot_field "ValueError: x and y arrays must have at least 2 entries". (need 2 yee cells around size=0 dimensions).
• at_centers() can give nan when interpolating at edges (wrap boundary conditions?)
• error when monitor size == simulation size, off by one discretization issue in loading
• downloaded data path can't be in 'monitor_data.hdf5', 'simulation.json' or 'tidy3d.log' because convert needs to download those files first. <- should be not a problem when integration is complete.
• Unique names still need ironing out, perhaps assign them on the server side instead of in validators.

[momchil-flex]

The first three should now be handled correctly in #100 with most of the heavy lifting done on the backend https://github.com/flexcompute/tidy3d-core/commit/c7abda6364583cd681dcac64b78153cce9e4ce44

P. S. This means that there won't be a noticeable change if you use the convert path and the old solver, but we're slowly getting ready to drop that.