Remove Edelman response function

[mbakker7]

The Edelman response function has been in the code for a while, but was never operational as it was not imported upon regular import of pastas.

The Edelman response function has problems, as it takes infinitely long before the step response reaches 1, and as a result it takes very long for it to reach the cutoff. Almost the same response can be obtained with the Polder function, which reaches 1 within a finite time.

I propose to remove the Edelman response function from the code base.

[rubencalje]

The Edelman function was proposed by Wilbert Berendrecht in 2019 (see your mail at the time), after which I committed it in Pastas. I experimented with it a bit at the time. The function is (or was) operational, but you can only reach it by pastas.rfunc.Edelman, and not directly by pastas.Edelman.

Wilbert maximized tmax at 20000, and he preferred it over the Polder-function, because the parameters of the Polder function were too correlated. Unless you think this is not true, or Edelman causes problems keeping the rest of pastas up to date, I am in favor of keeping it.

[dbrakenhoff]

Move to plugins when plugins API is complete.