Add demo to use external storage in DataJoint Matlab

matlab/external_storage/+external/Blob.m

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+ %{
+# This table is a toy example of how to save a blob field into the external location
+id:      int
+-----
+external_blob:     blob@imaging 
+%}
+
+classdef Blob < dj.Manual
+end

matlab/external_storage/+external/getSchema.m

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+function obj = getSchema
+persistent schemaObject
+if isempty(schemaObject)
+    schemaObject = dj.Schema(dj.conn, 'external', 'external');
+end
+obj = schemaObject;
+end

matlab/external_storage/set_up_dj_config.m

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+%% 
+% This notebook introduce how to set up the external storage path in the 
+% configuration variable |dj.config.|You could check the default of |dj.config| 
+% by:
+
+dj.config
+%% 
+% Now you can set up the host, user, and the password
+
+dj.config('databaseHost', 'host_name')
+dj.config('databaseUser', 'user_name')
+dj.config('databasePassword', 'password')
+%% 
+% We next would like to set up the external location. As an example, let's 
+% set up an external location for calcium imaging, which is named as |imaging|.
+
+imaging_storage = struct('protocol', 'file',...
+                         'location', '~/imaging');
+dj.config('stores.imaging', imaging_storage)
+%% 
+% Now the location has already set up. We could check the storage location 
+% by:
+
+dj.config('stores.imaging')
+%% 
+% We could save this config either locally with 
+
+dj.config.saveLocal()
+%% 
+% Or save globally with 
+
+dj.config.saveGlobal()
+%% 
+% Now let's create a schema and a table to test this location.
+
+dj.createSchema
+%% 
+% give a name to this new schema, for example |external|, and link it to 
+% a package named |+external|. Then inside the schema, create a table called |external.Blob| 
+% by
+
+dj.new
+%% 
+% In the file |+external/Blob.m|, we could see the definition of an external 
+% field to be |external_blob: blob@imaging|. |imaging| is the name of the storage 
+% we defined above. Now let's try insert 1 entry into this table:
+
+insert(external.Blob, struct('id', 1, 'external_blob', rand(10)))
+%% 
+%  | |Check the table contents:
+
+external.Blob
+%% 
+% Now fetch the |external_blob |as usual:
+
+fetch1(external.Blob, 'external_blob')
+%% 
+% To the user, it feels like the field is still stored in the database, 
+% but actually it is in the external storage, in this case ~/imaging.
+% 
+% If deleting the entry with |del| , the entry deleted is no longer fetchable, 
+% but the external files are not deleted unless you clean it up manually.
+
+del(external.Blob)
+%% 
+% To clean up, use the following command:
+
+% get the schema object
+schema = external.getSchema();
+schema.external.table('imaging').delete(true,[])
+%% 
+% 1st delete argument is bool for deleting from external source
+% 
+% 2nd delete argument is a number for a delete limit (optional)