Fix 375 and add support for writing trajectory frames to named files

[lohedges]

This PR closes #375 by implementing the missing SireIO::Gro87::getFrame() method. Previously, a null Frame() object would be returned whenever a frame loaded from a Gro87 file was accessed, e.g. via a TrajectoryIterator.

The PR also adds support for writing trajectory frames to user-defined filenames, rather than writing the output to a frame directory. This uses a frame_name map option, which then modifies the filename passed through to SireIO::MoleculeParser::writeToFile. This is mangling is required, since writeToFile is a static method. I've then wrapped the functionality in the new Sire API, allowing a user to pass a list of filenames to sire.save when the input (object to write) is a TrajectoryIterator. This allows the user to do things like:

import sire as sr

# Load a trajectory.
mols = sr.load_test_files("ala.top", "ala.traj")

# Write the first two trajectory frames to named files.
sr.save(mols.trajectory()[:2], ["cat/cat.gro", "dog/dog.gro"])
['/home/lester/Code/openbiosim/sire/trajectory_debug/cat/cat.gro',
 '/home/lester/Code/openbiosim/sire/trajectory_debug/dog/dog.gro']

A unit test validates that the correct output is generated, and that appropriate exceptions are thrown when the input is malformed, i.e. the number of filenames not matching the number of frames in the TrajectoryIterator, or there being a mismatch in the file extension between filenames.

Just to note the diff looks a bit weird owing to the removal of Gro87::operator[](int i), which now just returns an exception when used.

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[chryswoods]

Looks good. Sorry not to have implemented it - the migration to the frame-based way of working required stubbed implementations on all the older IO classes. The Gro87 class never got the stub implemented...

[lohedges]

No problem, it was very easy to add. I just wish I had thought to look there first, since it took me a while to backtrack my way via the trajectory code itself :-)