Fixes for various issues trying to set up first time

[mtauraso]

• Fixed passband caching system to use URL in filename, to avoid unwittingly using wrong locally cached files.
• Added passband file cache to .gitignore so git status doesn't report untracked files.
• Changed path in example ipynb to be relative to tdastro module path rather than hardcoded
• Fixed passbands.py to accept the LSST files from LSST github,
  • passbands.py now accepts a unit parameter for angstrom vs nm so LSST github files can be configured to work, as well as legacy files from svo2.cab.inta-csic.es
  • Fixed wavelength overlap condition and fluxes_to_bandflux() calculation so that longer passband tables from github work.

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Before [9fcdf6dd]	After [0441ab9e]	Ratio	Benchmark (Parameter)
6.92±0.05ms	6.86±0.03ms	0.99	benchmarks.time_chained_evaluate

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[mtauraso]

Looks good, thanks for adding this.

Let's discuss docstring style next Tues--no strong personal opinions, and I do really like how explicit the units parameter is here, but would be nice to codify something before we diverge too much.

I was a bit worried when I added that part... Not sure if it really even needs to be there, because I don't know much about what sorts of passband files tdastro needs to support.

I considered astropy.units for this part, but it seemed like too much complexity just to get the piece of code working again after I broke it by introducing the LSST passband file from github for seemingly the first time.

[mtauraso]

Planning to Merge sometime Tuesday (9/3) unless someone asks me not to.