PPSD: enable getting/plotting mode and mean psd values
Update outdated matplotlib URIs.
Update NumPy website.
Update official Python URIs.
Update SciPy links.
Update IRIS hosted URIs.
Update geopy URI.
Make PyPI links HTTPS.
Fix links to source files.
Update URIs that are redirecting.
Update URIs that are now 404.
Trigger length of 1 sample (on==off) resulted in empty array.
Bump NumPy version to match wheelhouse.
Merge pull request #884 from QuLogic/doc-linkcheck

Update doc links
Replaced if else with try Except to avoid unnecessary if checks.
Merge branch 'master' into ppsd_stats
minor refactoring
std of single value is always 0
minor docstring refactoring
Add ZMAP write support for earthquake catalogs