This is a dictionary of "known" names in nbody files.
It will be extended as the need arises.

The first few come from the headers:

float npart
float iter
float time
float ke
float pe
float masstot
float l_ang
float Gnewt
float epsilon
float zinitial
float omega
float hubble
float box_size
float znow
float initial_rsize
float initial_rmin_x
float initial_rmin_y
float initial_rmin_z
float veltime
float sizeof_ext
float alpha
float beta
float curvature
float lambda
float cosm_integral
float radius
int int_npart
int int_sizeof_ext
char tree_header_text[384];

The following arrays are of length (int)nbody.  Unfortunately,
in order to maintain historical compatibilty, the length of the
array is stored as a binary float in the files, so consider using
nbio_rd_double(..., "nbody") to figure out how long these vectors are.

float x[], y[], z[]
float vx[], vy[], vz[]
float mass[]

From the msw .ap files, we also have
float acc[]
float phi[]

Presumably, we should also reserve:
float ax[], ay[], az[]

The char id[][4] is specified by tree and tree_ap files.

There will undoubtedly be others.