Remove drho_dh

I doing two changes:

• I am removing rho_dh in PU
• Fixing an index in the theory document.

@jborrow you may want to look at it.

src/hydro/PressureEnergy/hydro.h

@@ -503,7 +503,6 @@ __attribute__((always_inline)) INLINE static void hydro_init_part(
   p->density.wcount = 0.f;
   p->density.wcount_dh = 0.f;
   p->rho = 0.f;
-  p->density.rho_dh = 0.f;
   p->pressure_bar = 0.f;
   p->density.pressure_bar_dh = 0.f;
 
@@ -537,7 +536,6 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
 
   /* Final operation on the density (add self-contribution). */
   p->rho += p->mass * kernel_root;
-  p->density.rho_dh -= hydro_dimension * p->mass * kernel_root;
   p->pressure_bar += p->mass * p->u * kernel_root;
   p->density.pressure_bar_dh -= hydro_dimension * p->mass * p->u * kernel_root;
   p->density.wcount += kernel_root;
@@ -545,7 +543,6 @@ __attribute__((always_inline)) INLINE static void hydro_end_density(
 
   /* Finish the calculation by inserting the missing h-factors */
   p->rho *= h_inv_dim;
-  p->density.rho_dh *= h_inv_dim_plus_one;
   p->pressure_bar *= (h_inv_dim * hydro_gamma_minus_one);
   p->density.pressure_bar_dh *= (h_inv_dim_plus_one * hydro_gamma_minus_one);
   p->density.wcount *= h_inv_dim;
@@ -590,7 +587,6 @@ __attribute__((always_inline)) INLINE static void hydro_part_has_no_neighbours(
   p->pressure_bar =
       p->mass * p->u * hydro_gamma_minus_one * kernel_root * h_inv_dim;
   p->density.wcount = kernel_root * h_inv_dim;
-  p->density.rho_dh = 0.f;
   p->density.wcount_dh = 0.f;
   p->density.pressure_bar_dh = 0.f;