Trajectory sampler: minor bug fix

[metdyn]

Types of change(s)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)
• Refactor (no functional changes, no api changes)

Checklist

• Tested this change with a run of GEOSgcm
• Ran the Unit Tests (make tests)

Description

This modification affects only the sampler code, especially the trajectory sampler. It fixes the following bugs:

• When group_name does not exist in netCDF file, the sampler code complains.
• The first time point was omitted when the model start time falls outside the time range for the trajectory. It is a valid setup.

Related Issue

I left some comments in the code, because I want to further test sampler code. More clean up will be performed afterwards.