further small improvements to numerical stability

improve lambda calculation
use Kahan summation to reduce FP error

should resolve INSIGNEO#52

make boolean case insensitive

fix spurious test failures

merge develop branch, adds masking support

fixes INSIGNEO#9

Merge branch 'develop'

allow intermediate map output

fix buggy hdf5 group name handling

group names are now specified with a separate string
fixes INSIGNEO#31

add appveyor badges

dcm loader segfault hotfix

fix broken tutorial.rst