Dataset caching

[davidhassell]

• Closes Issue Repeated reading of attributes is harming performance #8
• Tests added
• Changes are documented in CHANGELOG.rst

[bnlawrence]

What is going on. If you have a try with a raise attributerror and an except attributerror around it?

[davidhassell]

When the raise AttributeError happens, it jumps us to the except AttributeError: block, just as if a line of code had resulted in an AttributeError being raised.

[davidhassell]

Hi @bnlawrence - can we merge this, prior to making our PR the upstream h5netcdf? Ta

[davidhassell]

Hang on - I've noticed that this branch creates a failure in one of the h5netcdf unit tests (I presume the I overlooked this earlier, thinking that all the failures were the ones we expect and see on the main branch of h5netcdf).

I'm investigating. My initial thoughts are that if this is gnarly, we simply remove this PR from the final PR to upstream h5netcdf, especially because it is only a partial solution to the problem, and has comments in it along the lines of "I don't understand this, so I'm going to ...". Let's see it it's a simple fix ....