adding emdatabase as a dependency

[arthurmccray]

Adds em-database>=0.5 as a runtime dependency so that pip install quantem
is enough to run the tutorials.

Several tutorial notebooks (core/em_data.ipynb, diffractive_imaging/ ptycho_iter_04_MOSS6.ipynb, ptycho_iter_05_multislice.ipynb) import
emdatabase directly to download datasets and model weights. Until now users
had to know to install it separately.

Note the distribution is named em-database on PyPI; the import name is
emdatabase.

Also related: electronmicroscopy/quantem-tutorials#15