Removing the astroplan sites module (which migrated to astropy)

[bmorris3]

The astroplan sites module was added in #45, and then ported to astropy in astropy/astropy#4042. Since then, we've been meaning to remove the now redundant sites machinery from astroplan. In this PR, I removed the sites module and associated tests.

In addition, I had to create a new monkey patch. Many of the tests for the astroplan.Observer class used the class method astroplan.Observer.at_site to efficiently resolve coordinates for an observatory with the original sites module. Since the original sites module had a local copy of the site database stored offline, the tests written with astroplan.Observer.at_site would function identically in tests with or without the --remote-data flag. Now that we're using EarthLocation.of_site under the hood which always accesses the internet, I had to monkey-patch the EarthLocation.of_site method if the --remote-data option is not used, so that the tests can still resolve coordinates for the observatories in the tests without internet access.

This closes #121 and #40.

cc @cdeil, my monkey-patch tutor.

[bmorris3]

These tests will fail until we update the required astropy version to >1.1 (I think that's the version with EarthLocation.of_site). Since in #167 I raise the requirements to astropy>=1.2, we might as well wait to retest/merge this PR until #167 is merged.

[cdeil]

I think it's OK if astroplan 0.2 will require astropy 1.2.

[bsipocz]

@bmorris3 - this also closes #119

[ejeschke]

It would be nice if EarthLocation.of_site in astropy would not have to access the internet every time it is called. Is it not written to use a cache between process invocations, @bmorris3 ?

[bmorris3]

@ejeschke – the method caches the observatory database on first access, then accesses the cache each time unless it's forced to re-download, see source here. The monkey patch was needed because the tests would have to access the remote database once in order to download the database.

[bmorris3]

With the IERS errors fixed, this is ready for merge. This closes #119, #121 and #40.