Error handling with stats and files with different dimensions.

[faeglas]

Currently when using the stats command and a file_id with multiple files, the error thrown is this:

Traceback (most recent call last):
  File "./probtest.py", line 57, in <module>
    cli()
  File "/scratch/d1000/apps/buildbot/bb-venv/lib/python3.6/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib/python3.6/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib/python3.6/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/scratch/d1000/apps/buildbot/bb-venv/lib/python3.6/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib/python3.6/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/scratch/mch/fgessler/ICON/GLORI_test_small/CPU/externals/probtest/engine/stats.py", line 91, in stats
    file_specification,
  File "/scratch/mch/fgessler/ICON/GLORI_test_small/CPU/externals/probtest/engine/stats.py", line 15, in create_stats_dataframe
    df = df_from_file_ids(file_id, input_dir, file_specification)
  File "/scratch/mch/fgessler/ICON/GLORI_test_small/CPU/externals/probtest/util/dataframe_ops.py", line 150, in df_from_file_ids
    fid_dfs = unify_time_index(fid_dfs)
  File "/scratch/mch/fgessler/ICON/GLORI_test_small/CPU/externals/probtest/util/dataframe_ops.py", line 174, in unify_time_index
    df = df.reindex(columns=unique_times, level=time_multiindex_index)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/util/_decorators.py", line 309, in wrapper
    return func(*args, **kwargs)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/frame.py", line 4036, in reindex
    return super().reindex(**kwargs)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/generic.py", line 4464, in reindex
    axes, level, limit, tolerance, method, fill_value, copy
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/frame.py", line 3877, in _reindex_axes
    columns, method, copy, level, fill_value, limit, tolerance
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/frame.py", line 3925, in _reindex_columns
    allow_dups=False,
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/generic.py", line 4532, in _reindex_with_indexers
    copy=copy,
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/internals/managers.py", line 1285, in reindex_indexer
    self.axes[axis]._can_reindex(indexer)
  File "/scratch/d1000/apps/buildbot/bb-venv/lib64/python3.6/site-packages/pandas/core/indexes/base.py", line 3292, in _can_reindex
    raise ValueError("cannot reindex from a duplicate axis")
ValueError: cannot reindex from a duplicate axis

We could make a simple check to make sure all files a user is trying to use stats on have the same dimensions and give a more meaningful and lightweight error message. E.g: "Error: File a and File b have different dimensions. They must have the same dimensions."