Generalize the derivative matrix

Wicked fast, see benchmark file added in this commit:

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multifluid derivatives
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/Users/ihb/Documents/Code/teqp/src/bench_derivbox.cpp:37
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benchmark name                       samples       iterations    estimated
                                     mean          low mean      high mean
                                     std dev       low std dev   high std dev
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alphar                                         100           140      1.652 ms 
                                        117.036 ns    116.863 ns    117.654 ns 
                                        1.48691 ns   0.425603 ns    3.42119 ns 
                                                                               
All residual derivatives needed for                                            
first derivatives of h,s,u,p w.r.t.                                            
T&rho                                          100            29     1.6849 ms 
                                        579.424 ns    575.931 ns    593.892 ns 
                                        30.0729 ns    2.63483 ns    69.5014 ns 
                                                                               
All ideal-gas derivatives needed                                               
for first derivatives of h,s,u,p w.                                            
r.t. T&rho                                     100            72     1.6416 ms 
                                        226.557 ns    226.297 ns    227.014 ns 
                                        1.70744 ns    1.13538 ns    3.15493 ns 
                                                                               

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