Replace Module name from opmd_viewer to openpmd_viewer

[RemiLehe]

As discussed with @ax3l and @soerenjalas, in version 1.0, we will replace

import opmd_viewer

by

import openpmd_viewer

This is what the current PR does, by:

• git mv-ing the whole opmd_viewer folder to openpmd_viewer
• sed-ing all relevant files to automatically change opmd_viewer into openpmd_viewer

There will also be a later PR that leaves a stub, so that importing opmd_viewer will raise an Exception (prompting the user to either downgrade to version 0.X or to change the import syntax.)

[ax3l]

Thx! Just a small typo left.

I renamed the PR title accordingly, since the "python import"-name is the "module" name. The package name (#223) would be the PyPI (already correct), spack (todo) or conda(-forge) name (todo when I am back).

Ref.s:

• Python Import Name openPMD-api#181 (comment)
• close Consistent module naming: openpmd_viewer #82