Add performance tests

[khaeru]

Use pytest-benchmark or similar tools to test performance metrics.

The ixmp unit tests use simple models like the Dantzig cannery/transport problem to quickly test basic features. However, ixmp is also used (via message_ix) for MESSAGE-GLOBIOM, in which parameters have up to 14 indices and ~2 million rows. Speed and memory performance is important for these data.

The metrics should include the following:

• Read time per row. While testing Add CachingBackend, address performance of JDBCBackend.item_get_elements #213, @OFR-IIASA stated that 50 seconds to load the land_out parameter with ~1.94 million rows is “reasonable”, while 470 seconds is not. These numbers correspond to 26 and 242 µs / row, respectively.
• Write time per row. In Performance: Reporting upload of timeseries  #66, @OFR-IIASA reported a time of 40 minutes for certain data as unacceptable; but the size of the data was not stated.

Miscellanea:

• It may be difficult to test performance issues due to network speeds and the Oracle database server used by IIASA ENE. e.g. @OFR-IIASA reports that TimeSeries.commit() can be extremely slow in some cases.

[khaeru]

cf. #227 and #298 for memory performance.