[BUG] Issues with stats output

[MGrehan]

The three following problems have been found with the stats outputs:

• The time and step values are incorrect. Easy to show, run simulation with runtime = 1.0, then read the stats and you will see that the final time value is nowhere near 1.0. Similarly, the step number does not match when using data.attrs['output.stats.interval_time']. Current work around is using data.attrs['output.stats.interval_time'] and making new time array based off index of array, so seems to be outputting at right times but steps and time are wrong values.

• T00 in stats is not in units of m0 n0, seems to be related to the sum of particle Lorentz factor, but off by factor of 2 when testing?

• quantities labels does not work for some values, eg E2 and Npart give errors as "Unrecognized stats name" (while E^2 works...)

[haykh]

Unable to reproduce the 1st one. if i run with interval_time = 0.2, i get this (which is correct):

          step,          time,           T00,          E1^2,          E2^2,          E3^2,         Npart,
             1,    0.00690534,   4.37968e+06,    1.0093e-11,   1.01064e-11,   8.93979e-12,    4.1943e+06,
            30,       0.20716,   4.37968e+06,   4.79245e-09,   4.79713e-09,   5.54013e-10,    4.1943e+06,
            59,      0.407415,   4.37968e+06,    1.7173e-08,   1.72043e-08,   6.69523e-10,    4.1943e+06,
            88,       0.60767,   4.37968e+06,   3.57332e-08,   3.58585e-08,    7.2639e-10,    4.1943e+06,
           117,      0.807925,   4.37968e+06,   5.83172e-08,   5.85327e-08,   7.32685e-10,    4.1943e+06,

also works ok if i supply interval = 20 (i.e., every 20th timestep).