Fixes the compilation problems, I hope

Python part still needs to be fixed, almost there, compiles in clang again

To be sure

This is why you should make sure it compiles...

Closing in

Very fast in C++ (less than a microsecond for 4 outputs in the Jacobian), but 10 microseconds in Python for 2. Need to find a way to make it closer to original speed. Perhaps pass in a buffer to be populated.  That should allow the copy to be avoided. It must be the Eigen arrays that are so slow.

See #21

Wicked fast, see benchmark file added in this commit:

-------------------------------------------------------------------------------
multifluid derivatives
-------------------------------------------------------------------------------
/Users/ihb/Documents/Code/teqp/src/bench_derivbox.cpp:37
...............................................................................

benchmark name                       samples       iterations    estimated
                                     mean          low mean      high mean
                                     std dev       low std dev   high std dev
-------------------------------------------------------------------------------
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 
                                                                               

===============================================================================

Oddly this fixes the error in derivative, but it should have no impact

Temporaries were being returned when autodiff was being used, forceeval should fix it I think

It works!

* Add partial molar volume as a possible output

* And expose to python

To avoid unused variable warnings

By adding the necessary Eigen header for real type of autodiff

Next is the cross derivatives