Add second order partial derivatives w.r.t. parameters

[TiKeil]

In this PR, I expand the first order derivatives w.r.t parameters as done in #748 to second order partial derivatives.

I more or less edited the same amount of code as in #748 and I adapted the test to it.

I went for the name "second_derivative". Another idea was to call it hessian, but this is a little bit dependent on the application I guess. With the code, it is also possible to only give a few expressions and not all.

Any objections ?

[sdrave]

@TiKeil, can you rebase on current master? This should hopefully make CI pass.

Can you argue why it is better to have a dedicated method for the second derivatives instead of calling d_mu twice?

[codecov]

Codecov Report

Merging #840 into master will increase coverage by 0.03%.
The diff coverage is 91.3%.

Impacted Files	Coverage Δ
src/pymor/parameters/functionals.py	93.75% <91.3%> (+1.19%)	⬆️

[TiKeil]

Can you argue why it is better to have a dedicated method for the second derivatives instead of calling d_mu twice?

This is actually a good point. I think there is no particular reason for having a second method although I find 'd_mui_muj' a little bit more intuitive to call.
However, I could change this PR by removing 'd_mui_muj' ?

[TiKeil]

I removed d_mui_muj in f74b059
Any more objections?

[sdrave]

LGTM.

[TiKeil]

I added the changes and rebased on master