#102 Remove/Encapsulate boost::math (removing boost #1)

[Ulf Lorenz]

Boost is a major drag when you want to compile anything, as the huge number of headers slows down the compilation considerably.

In this issue, we want to remove one dependency, namely boost::math. It should only be referenced in a handful of special files (special_functions.cpp, maybe something like special_constants.cpp), everything else should reference a forward from the corresponding header files.

• add forwards for all used special functions to special-functions.*
• add forwards for all used constants either in special_functions.* or a separate special_constants.*
• remove all includes for boost::math

Measure the effect of this cleanup on compile times!

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Added an own helper function to get value of pi only. Everything else was strongly localized. The effect on compile times was negligible (~ 1%)