Be able to chunk the display of the tree

[RoyLarson]

I have some logs with really long tasks, that do not allow me to scroll all the way through the history. I would like to be able to iterate the the log lines some chunk size at a time. like

(pipeline_opt) λ eliot-tree logs/2020-08-19_0004.log -l 0 --chunk-size 10
2450eced-dde4-477d-96fc-64409a0b371f
└── pipeline_optimize.initialization.initialize_tanks.new_tank/1 ⇒ started 2020-08-19 23:33:03Z ⧖ 13.326s
    ├── fac: InletFacility(InletFacTag(site='SANEAST2', service='UIS', facility_name='THIS_FAC', property_num='121312'), [PumpFacility(FacilityTag(site='SANEAST2', service='UIS', facilityid='THIS_FAC_LACT'))]
    ├── initialization_days: 7
    ├── log_level: 30
    ├── pipeline_optimize.services.cygnet_services.cygnet.get_current_values_for_facility/2/1 ⇒ started 2020-08-19 23:33:03Z ⧖ 0.409s
    │   ├── fac: PumpFacility(FacilityTag(site='SANEAST2', service='UIS', facilityid='THIS_FAC_LACT'))
    │   ├── log_level: 30
    │   ├── pipeline_optimize.services.cygnet_services.cygnet.load_current_value/2/2/1 ⇒ started 2020-08-19 23:33:03Z ⧖ 0.017s
    │   │   ├── log_level: 30
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