Patch auto-download executables

[bdestombe]

• download_mfbinaries() should not retreive any None values for version_tags. Which was the case if: no mf6 was ever downloaded on your system and no version_tag was specified in get_bin_directory() or to_model_ds() .
• Remove explicit download commands and rely on auto download of executables. Closes Remove explicit download of executables command from examples #348

[bdestombe]

Uhhg. They have built a nasty conditional statement in flopy that detects testing environment and in case of testing don't write the metadata. The written metadata is needed later for locating installs. https://github.com/modflowpy/flopy/blob/0748dcb9e4641b5ad9616af115dd3be906f98f50/flopy/utils/get_modflow.py#L623

A bit like the Volkswagen/audi/diesel gate that detected when the cars were tested for emissions. During testing conditions they would emit less..