s_sizer field to control multiple blocks simultaneously

[ajohnston9]

I'm trying to utilize a s_size field that will control the length of two fields simultaneously, but only appear once in the message. Right now, I can see that it only takes one block name.

If this fix is as simple as plugging in a for loop in the definition of s_size, I'm happy to submit a PR. That said, I imagine it would likely introduce problems somewhere else in the framework, so I wanted to file the issue before getting started.

[SR4ven]

Being able to run a sizer over multiple blocks would indeed be a nice feature.

I'm not sure what problems we'll face implementing this. A simple for loop alone might not do it. I'm thinking of a case where s_size is part of one of the blocks it calculates the size of.

However, I guess the only way to find out what difficulties we'd face is to start implementing it. The unit tests should notify you of any unexpected behaviour. So a PR is always welcome.