BUG: poor memory management preventing full test of concat_data

[aburrell]

Describe the bug
GitHub Actions cannot run for the operational test, due to poor memory management.

To Reproduce
Steps to reproduce the behavior:

1. Go to pysat/pysat/tests/classes/cls_instrument_library.py
2. Change the pytest.skip conditions to allow Python 3.6 to run for the test_load_w_pad test.
3. Perform the full unit test suite.
4. See error

Expected behavior
These tests will pass without the full suite locally and the behaviour is correct in iPython. The full test suite should run.

Desktop (please complete the following information):

• OS: GitHub Actions Ubuntu
• Version: Python 3.6
• Other details about your setup that could be relevant

Additional context
The error we are getting is that too much memory is being used. I was able to significantly reduce memory usage in Instrument.load by making changes to __getitem__. It is likely that the use of __slots__ (which requires removing weakref usage), and improving other algorithmic redundancies can solve this problem.

[aburrell]

We should probably establish benchmarking for whatever bits of code we start working on for this to better track improvements (see #811).