cryptic error message for plot_tfr

It sometimes complains
```
TypeError: Dimensions of C (10000, 160) should be one smaller than X(10000) and Y(160) while using shading='flat' see help(pcolormesh)
```
The culprit might be the shape of `values` is backwards from what is expected. We should check that C has shape (len(Y), len(X)) and make a warning if not. I think the shape can also be off by one and it's ok, it just uses centers instead of edges for shading.