Temperature warnings need changing

[timcallow]

A warning is currently issued if the temperature is outside of the range [0.01, 3.5] Hartree. A message is then printed that the "normal temperature range for atoMEC is 0.01 - 100 eV". There are two problems here:

1. The printed range does not actually match correctly to the range which triggers the warning
2. 100 eV is a bit of a random choice. We can certainly do temperatures that high, indeed we can often reach up to 1000 eV. It depends on the density.

I think we should essentially scrap these warnings. Somewhere in the documentation we should include a section on calculation issues in which we can mention the conditions under which atoMEC struggles, and possible strategies in those cases,