avoid masking exceptions that occurred during image comparison tests
(and that lead to an empty image file getting read in comparison,
raising a misleading exceptions)
add context manager that catches stdout/stderr into string variables,
works even when C level code is writing to stdout/stderr.
catch C level stdout in a GSE2 test to unclutter obspy-runtests output

see #653, see
https://github.com/obspy/obspy/pull/653#issuecomment-26633094
suppress some warnings in tests
avoid numpy runtime warning, much cleaner this way
warnings get cleaned up after catch_warnings() automatically, this was
influencing global warning filters
avoid some numpy warnings in mopad (see geophysics/MoPaD#3)

handling could be improved there, it seems some calculations for valid
output rely on nans getting produced via e.g. zero divisions.
catch some more warnings in tests, obspy-runtests output is free from warnings again
add missing submodules to docs. we should maybe handle this differently
so that new submodules can't be forgotten in docs