update Singularity to new omuse directory structure

[ipelupessy]

once we have a new version to release, the Singularity build def should be updated to use new omuse install procedure/ directory structure

minimally, by replacing:

cd src

hg clone -r v1.1 https://bitbucket.org/omuse/omuse

with:

cd ..
hg clone https://bitbucket.org/omuse/omuse
cd amuse
cd src
ln -s ../../omuse omuse

but probably more extensive simplifications are necessary