Attempt to add functionality to use singularity

This mostly follows instructions from computing.docs.ligo.org as the recommended way to maintain stable custom environments over the OSG.

• https://computing.docs.ligo.org/guide/htcondor/credentials/?h=scitok#__tabbed_2_1
• https://computing.docs.ligo.org/guide/htcondor/software/#transfer-singularity

We previously had some singularity support, but it was never well integrated, possibly due to immaturity of the support through HTCondor.

I also made a start on porting some of the additional jobs to use the grid.

TODO:

• check authentication
• write documentation
• run a test?