feat(poly): added NonlinearPowerIntegrator and PolytropeOperator

A custom integrator is required to handle the theta^n term in the lane emden equation, that is written as NonlinearPowerIntegrator which is a mfem::NonlinearFormIntegrator and defines methods to assemble its element vector (function value) and element gradient matrix (jacobian). This is then, along with built in mfem vectors for M Q and D, incorporated into the PolytropeOperator which defines methods for Mult (calculate the residuals of the variational form) and GetGradient (find the jacobian of the system)

src/meson.build

@@ -3,6 +3,7 @@
 # as there are dependencies which exist between them.
 
 # Utility Libraries
+subdir('types')
 subdir('misc')
 subdir('config')
 subdir('probe')
@@ -21,3 +22,4 @@ subdir('resource')
 
 # Physics Libraries
 subdir('network')
+subdir('poly')