trying to fix weird race condition on windows causing the solver window to close when closing the message window

what was causingthe uncomprehensible LNK1179 error ("duplicate
COMDAT") with MSVC.

No idea why.

moved all string options to std::string

*****************************************************************
WARNING: this has the potential to break things in subtle ways...
         PLEASE TEST AND REPORT ANY WEIRDNESSES
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get string option would NEVER work! Ugly fix until will replace all const char*
with std::strings in the options

Uniformisation of the weights of the Gauss Quadrature points (sum = volume of the reference element)
Adding Gauss Quadrature rule for a line
Modifications by Gaetan Bricteux