REST API wrapper for the processor

[tdoan2010]

Description

This pull request provides the REST API wrapper for the processor. Instead of "one-shot mode", the processor now can be started as a server. There are 2 ways to start the processor:

1. Use processor name. This is the recommended way to start the Pythonic processor as a server.

# Syntax
<processor-name> --address=IP:PORT:MONGO_URL

# Example
$ ocrd-dummy --address=0.0.0.0:80:mongodb://localhost:27017

2. Use ocrd tool. This approach is for non-Python processor.

# Syntax
ocrd processing-server <processor-name> --address=IP:PORT:MONGO_URL

# Example
$ ocrd processing-server ocrd-dummy --address=0.0.0.0:80:mongodb://localhost:27017

Options

1. processor-name: executable name of the processor
2. --address: it has 3 parts, separated by colon, IP:PORT:MONGO_URL, where
   1. IP: the IP address that the server should listen on.
   2. PORT: the port that the server should listen on.
   3. MONGO_URL: the full connection string to a running Mongo database instance.

[kba]

Why is output_file_grp optional but input_file_grps is not?

[tdoan2010]

Is there a case when a processor can run without input file groups?

[joschrew]

Hi, I am/was not able to call a processing server running for ocrd-olena-binarize (a bashlib processor). I think it is because of a bug in the processing server in helpers.py in function run_cli_from_api. In line 196 and 197, run_cli is called with mets_url=''`` which should be changed to mets_url=workspace.mets_target` or at least that resolved my problem.
To reproduce I did nothing special, just run the server as described and call it as described.

[tdoan2010]

@joschrew thank you for your comment. I pushed the fix as you suggested.