Better warnings and consistency tests of the df module

In this MR we make sure to provide descriptive error messages for the user when entering one of several special cases in the dielectric function module. As an example, we advice the user to calculate TDDFT dielectric properties via the inverse dielectric function in order to avoid erroneous results due to #1180 (closed).

We also start testing the up-coming user-interface in gpaw.test.response.test_diamond_absorption, including the TDDFT absorption, and test that the resulting macroscopic dielectric function is the same in RPA no matter whether the user calculates it via the dielectric function, the bare dielectric function or the inverse dielectric function.

Replaces !2310 (closed) Closes #1180 (closed)