Clean up of the XSPEC interface code

[DougBurke]

Summary

Internal changes to the XSPEC interface code, which reduces the amount of similar (sometimes identical) code. There is no change to the behavior of the XSPEC models.

Details

Noted by @dtnguyen2 in a review of #772

• Switch to using std::transform in XSPEC Fortran-style models
• Reduce duplication (but there is more, eg The 4 functions in xspec_extension.hh file can be reduced to 2, if C++ template code is written properly #860)

I'm not likely to work on this anymore (modulo PR review) so we can either review this or through it.

[DougBurke]

I've rebased this because of changes to the XSPEC interface (for XSPEC 12.11.0 support).

[DougBurke]

It appears that this code barfs on the 98 C++ standard, which we still want to support easily (i.e. without forcing explicit calls to a newer standard).

[dtnguyen2]

One could if def our way out of this mess:

#include <iostream>

int main(){
#if __cplusplus==201402L
  std::cout << "C++14" << std::endl;
#elif __cplusplus==201103L
  std::cout << "C++11" << std::endl;
#elif __cplusplus==199711L
  std::cout << "C++98" << std::endl;  
#else
  std::cout << "C++" << std::endl;
#endif
  
  return 0;
}

(base) [dtn@devel12 dtn]$ g++ -std=c++98 --pedantic tmp.cc; a.out
C++98
(base) [dtn@devel12 dtn]$ g++ -std=c++11 --pedantic tmp.cc; a.out
C++11

[wmclaugh]

@DougBurke - I agree with Dan's suggestion of a conditional compile here to support the c++ 98 standard as well as the c++ 11 standard. The 11 standard provides improved performance and compatibility with other libraries built against the standard. Whether we would want to eventually drop the 98 standard is a separate issue as is the infrastructure changes to support specifying/propagating (commandline) compiler flag options.