QuakeML: correctly parse amplitudes
quakeml: fix reading time_window in amplitude
quakeml: test for reading amplitude / time window
quakeml: fix helper method to write time to xml, was disregarding
microseconds, simply relying on `UTCDateTime.__str__` is totally fine
quakeml: write amplitudes to quakeml + basic test
Merge remote-tracking branch 'upstream/releases' into fix_qml_amplitudes
Merge remote-tracking branch 'upstream/releases' into fix_qml_amplitudes
adapt some quakeml test files

only cosmetic changes, easier to modify the test file than adapt testing routine
bug fixes in quakeml Amplitude I/O

triggered by a suddenly failing mchedr test (writing to quakeml.
@claudiodsf: beware that anything so far converted from mchedr to
quakeml got stripped of amplitudes during conversion!)
quakeml: don't fill in empty WaveformStreamID (but None) during parsing
Merge pull request #763 from obspy/fix_qml_amplitudes

QuakeML misses I/O for Amplitude(s)