Remove GEVMethod and H2 from optimizer

[markdewing]

The GEVMethod parameter (apparently has values 'mixed' or 'H2') is unused and not documented. (Are there any publications describing it?)

When increasing the number of variational parameters, the fillOverlapHamiltonianMatrices function eventually becomes a bottleneck. This PR is a move towards simplifying that function and reducing the number of operations.

What type(s) of changes does this code introduce?

Delete the items that do not apply

• Other (please describe): Remove unused feature

Does this introduce a breaking change?

• Yes

What systems has this change been tested on?

desktop

Checklist

Update the following with a yes where the items apply. If you're unsure about any of them, don't hesitate to ask. This is
simply a reminder of what we are going to look for before merging your code.

• Yes. This PR is up to date with current the current state of 'develop'
• Yes. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• No. Documentation has been added (if appropriate)

[rcclay]

I might need a day to work through this and see if I can figure out what the intention of the H2 stuff was.

[rcclay]

Took me ages, but had a chance to look at this. Undocumented feature, probably a remnant from the days that linear method was first implemented. I can't find any obvious references to this in the literature... there might be something from before Umrigar and Toulouse finalized their algorithm, but I don't think it's worth pursuing. LGTM.

[prckent]

Many thanks Mark. Another ghost from the past removed.

[prckent]

Test this please