Create command to show available Schedulers

[victorgarcia98]

Following on the creation of the command flexmeasures show reporters (PR #686), this could be replicated for the Scheduler class. This command should look something like this:

Command Syntax

$ flexmeasures show schedulers

Example Output

Schedulers:

name                   version         author          module
--------------        ---------        --------       ----------------------------------------
StorageScheduler        1                             flexmeasures.data.models.planning.storage

This command should list all the schedulers available from the flexmeasures and from all the plugins registered to the instance, as well.