Make and document an IMD server for serving written trajectories via IMD

[ljwoods2]

Is your feature request related to a problem?

The InThreadIMDServer serves an MDA Universe in pure Python for testing, but it has some test-specific methods like expect_header and has some placeholder data specifically for testing (like in the energy packet).

Describe the solution you'd like

A more generalized class could be made similar to InThreadIMDServer for the use case of serving a prewritten trajectory via IMD.

Describe alternatives you've considered

Don't do anything

[orbeckst]

Could be IMD.IMDServer (kind of alternative to IMDWriter ... or call it IMDWriter because that's really what you're doing: write to an IMD stream).

[orbeckst]

And then you can use MDA as a stream filter: read from an IMD stream and write out to another one that is then used by e.g., VMD. Interesting possibilities!

[ljwoods2]

Great idea, the writer interface is natural for this and error handling will be much easier since it has a context manager. I like the idea of being able to optionally use v2 to write to VMD as well.