enable reading of gzipped instance catalog components and includeobj entries

[jchiang87]

This will be more consistent with phosim behavior and will make the code to write instance catalogs for both imsim and phosim simpler.

[cwwalter]

How do you see this interacting with #71?

[jchiang87]

I think getting imsim able to read the same format files as phosim wrt gzipped files and includeobj is a more pressing concern than the work in #71. Adding the desired functionality here will result in much less work in instance catalog generation code. #71 isn't as relevant.

[cwwalter]

For future reference for people this issue (raised by Tom, answered by John) has an explicit description of the functionality.

https://bitbucket.org/phosim/phosim_release/issues/6/problem-interpreting-gzipped-instance