Minor improvements: improved voyager notebooks + added conda installation instructions

[zachtheyek]

This PR contains the following changes:

1. In examples/voyager.ipynb and tutorial/blimpy_voyager_tour.ipynb, moved %matplotlib inline call to after the blimpy import.

• This was motivated by the observation that blimpy resets the matplotlib backend to agg on import. To verify, in a fresh kernel, run:

%matplotlib inline
import matplotlib
print("Backend right after magic:", matplotlib.get_backend())

import blimpy     
print("Backend after importing blimpy:", matplotlib.get_backend())

• This is important, since agg is a non-interactive, file-only renderer. In other words, it prevents plots from being displayed to the notebook/screen.
• %matplotlib inline resets the backend to module://matplotlib_inline.backend_inline, but the previous sequencing was in the wrong order.

2. In examples/voyager.ipynb and tutorial/blimpy_voyager_tour.ipynb, changed the "get data" portions to first check whether the data exists, and if not, download it to the CWD.

• Pretty straightforward change aimed at blimpy beginners by making quality of life better when running the notebooks out-of-the-box.

3. Added an environment.yml file containing the same dependencies listed in requirements.txt, alongside setup instructions in README.md.

• Doesn't affect core functionality, just gives users another option for setting up blimpy.

[zachtheyek]

edit: i suspect the %matplotlib inline change (#1) is related to #279

imo if there's no reason to reset the matplotlib backend, a longer-term solution would be to remove that behavior from the core blimpy functionality altogether. perhaps it was unintentionally set to agg somewhere where figures are saved / loaded into memory? just a hunch