Replace std::pow where possible

[gassmoeller]

I saw by chance that we still have a number of places in the code where we do std::pow(x,2). This is pretty slow (similar to the cost of allocating a std::vector), and although most of the occurrences are not performance critical, at least a few are.
This PR is a bit of regular expression substitutions magic and a bit of manual substitutions. I think I replaced all cases of std::pow with:

• Utilities::fixed_power if the exponent is integer and known at compile time
• Utilities::pow if the exponent is integer and known at run time

[gassmoeller]

Ah, it looks like Utilities::pow only supports double as a base after deal.II 9.5 thus the tester fails. Will get back to this.

[gassmoeller]

It turns out Utilities::fixed_power, Utilities::pow and std::pow return (marginally) different results due to floating point accuracy and the used algorithms. I had to update a few test results, although all of them only cause small changes in iteration counts or final residuals. I am pretty certain this is not a problem for practical models. @tjhei are you still ok with the change?