feat(poly): added skeleton of polytrope model

the polytrope module will be used as an initial guess to the solver. A skeleton of this has been imported from https://github.com/tboudreaux/FEMPolytrope

This module will need major updates still to handle 3D, proper boundary conditions, and to incorporate it with the rest of our meshing scheme

src/poly/coeff/meson.build

@@ -0,0 +1,23 @@
+polyCoeff_sources = files(
+  'private/coeff.cpp'
+)
+
+polyCoeff_headers = files(
+  'public/coeff.h'
+)
+
+libPolyCoeff = static_library('polyCoeff',
+  polyCoeff_sources,
+  include_directories : include_directories('.'),
+  cpp_args: ['-fvisibility=default'],
+  dependencies: [mfem_dep],
+  install: true
+)
+
+
+polyCoeff_dep = declare_dependency(
+  include_directories : include_directories('.'),
+  link_with : libPolyCoeff,
+  sources : polyCoeff_sources,
+  dependencies : [mfem_dep]
+)