Support Galaxy Expression Tools in Pulsar

[jmchilton]

Need to transfer the Python script - perhaps as a config file or tool file.

[jmchilton]

exec_before_job writes files Pulsar doesn't know about and don't seem to respect compute environments:

        local_working_directory = param_dict["__local_working_directory__"]
        expression_inputs_path = os.path.join(local_working_directory, ExpressionTool.EXPRESSION_INPUTS_NAME)

The cd ../ is a bit questionable also.

        self.command = f"cd ../; {expressions.EXPRESSION_SCRIPT_CALL}"