Add compute_pressure_cylinder documentation

doc/src/compute_pressure_cylinder.txt

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+"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
+
+:link(lws,http://lammps.sandia.gov)
+:link(ld,Manual.html)
+:link(lc,Section_commands.html#comm)
+
+:line
+
+compute pressure/cylinder command :h3
+
+[Syntax:]
+
+compute ID group-ID pressure/cylinder zlo zhi Rmax bin_width :pre
+
+ID, group-ID are documented in "compute"_compute.html command
+pressure/cylinder = style name of this compute command
+zlo = minimum z-boundary for cylinder
+zhi = maximum z-boundary for cylinder
+Rmax = maximum radius to perform calculation to
+bin_width = width of radial bins to use for calculation :ul
+
+[Examples:]
+
+compute 1 all pressure/cylinder -10.0 10.0 15.0 0.25 :pre
+
+[Description:]
+
+Define a computation that calculates the pressure tensor of a
+system in cylindrical coordinates, as discussed in "(Addington)"_#Addington1.
+This is useful for systems with a single axis of rotational symmetry,
+such as cylindrical micelles or carbon nanotubes. The compute splits 
+the system into radial, cylindrical-shell-type bins of width bin_width, centered at x=0,y=0,
+and calculates the radial (P_rhorho), azimuthal (P_phiphi), and axial (P_zz) components of the configurational pressure
+tensor. The local density is also calculated for each bin, so that
+the true pressure can be recovered as P_kin+P_conf=density*k*T+P_conf.
+The output is a global array with 5 columns; one each for bin radius,
+local number density, P_rhorho, P_phiphi, and P_zz. The number of 
+rows is governed by the values of Rmax and bin_width. Pressure
+tensor values are output in pressure units.
+
+[Output info:]
+
+This compute calculates a global array with 5 columns and Rmax/bin_width
+rows.
+
+The values calculated by this compute are
+"intensive".  The pressure values will be in pressure
+"units"_units.html. The number density values will be in 
+inverse volume "units"_units.html.
+
+[Restrictions:] none
+
+[Related commands:]
+
+"compute temp"_compute_temp.html, "compute
+stress/atom"_compute_stress_atom.html,
+"thermo_style"_thermo_style.html,
+
+[Default:] none
+
+:line
+
+:link(Addington1)
+[(Addington)] Addington, Long, Gubbins, J Chem Phys, 149, 084109 (2018).