GROMOS11 reader is slow

[schuhmc]

Is your feature request related to a problem?

The current implementation of the GROMOS11 reader is relatively slow. See also:

There were some possible improvements identified for the reader in the review on PR #4292

• performance improvements (reader itself and when used in CHAINREADER)

• have MDA internally convert special GROMOS11 PBC treatments to triclinic

Please feel free to raise separate issues that could then be addressed separately.

Originally posted by @orbeckst in #4291

Describe the solution you'd like

Significant improvement of the read speed of GROMOS trajectories.

Describe alternatives you've considered

None

Additional context

I will implement this feature.