`numba` is also a dependency

[berceanu]

It seems numba should be added to the requirements file. Also, openpmd_viewer 1.X uses from openpmd_viewer import .. instead of from opmd_viewer import .., as stated in the README

[hightower8083]

hi @berceanu

yes this part with numba import is assumed but is not correct. Actually numba is only used for a couple of secondary methods in converters and is not necessary for any actual synchrad's functionality. I would prefer to keep numba dependency optional -- it's a big one and has its own specific relations to numpy. I've added handling of not-installed case with a warning