Need to assert that deephaven_server.Server is started before importing deephaven

[chipkent]

When using Deephaven as a Python library, the deephaven module can only be safely imported after the deephaven_server.Server has been started. A check needs to be added to deephaven/__init__.py to ensure the server has been started.

The bug was introduced in v0.16. In versions before v0.16, the server needed to be created (to create the JVM), but the server did not need to be started. Changes related to Python variable scoping in the query language introduced the new requirement that the server be started.

[devinrsmith]

Here is the new error:

>>> import deephaven_server
>>> server = deephaven_server.Server()
# Starting io.deephaven.python.server.EmbeddedServer
# Bootstrapping from resource '/bootstrap.properties'
# io.deephaven.internal.log.LoggerFactoryServiceLoaderImpl: searching for 'io.deephaven.internal.log.LoggerFactory'...
# io.deephaven.internal.log.LoggerFactoryServiceLoaderImpl: found 'io.deephaven.internal.log.LoggerFactorySlf4j'
>>> import deephaven
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/devin/.venvs/deephaven-server-0-16-1/lib64/python3.10/site-packages/deephaven/__init__.py", line 19, in <module>
    from .table import SortDirection, AsOfMatchRule
  File "/home/devin/.venvs/deephaven-server-0-16-1/lib64/python3.10/site-packages/deephaven/table.py", line 54, in <module>
    _j_script_session = jpy.cast(_JExecutionContext.getContext().getQueryScope(), _JUnsynchronizedScriptSessionQueryScope).scriptSession()
AttributeError: 'NoneType' object has no attribute 'scriptSession'
>>> server.start()
Server started on port 10000
>>> import deephaven
>>> 

[devinrsmith]

I think ultimately, this is due to the introduction of java execution at python module initialization time, something we haven't done before (up until now, we have mainly just used jpy.get_type, or accessing constant values). I've got a fix I'll get up for review soon.