Additional fix for missing pandas.Panel

[enricodetoma]

I made an additional fix to manage the case of missing Panel in new pandas versions.
In fact I discovered that in few cases (e.g. library.delete(symbol)), PandasPanelStore static method can_write_type was still called.
With this commit, PandasPanelStore is not even registered as storage class if Panel is missing from pandas.

[shashank88]

@enricodetoma you need to rebase off the latest master after pinning pandas and numpy

[enricodetoma]

@shashank88 I rebased off the latest master, but I'm not sure what you mean about pinning pandas and numpy.
This pull request completes my previous pull request #881 which was meant to allow to use Arctic with recent pandas versions, though without Panel support (but then without the need to pin it to a specific version, I guess).

[enricodetoma]

Actually I'm already using Arctic successfully with pandas 1.2.3 with this change, without Panel support

[crazy25000]

Actually I'm already using Arctic successfully with pandas 1.2.3 with this change, without Panel support

Same here, using it with latest Pandas. I don't use panels explicitly, just using tick store. Chunk store too slow to use.

[samuelrohr]

Any update on this review?

[dunckerr]

Hi Enrico - we haven’t removed Panel support because we are still supporting old versions of Pandas. Am I correct that you’re forking arctic repo and removing Panel support? We are always interested to hear about any improvements to the arctic codebase.
Thanks
Duncan

[enricodetoma]

Hi Duncan, I'm simply skipping importing Panel if it doesn't exist:

try:
    from pandas import Panel
    register_versioned_storage(PandasPanelStore)
except ImportError:
    pass