Removing .entries usage from slhaplot

Typo fix

Adding a dict arg (compatible with dict.update) to the Block constructor.

Adding Block.set_value(*args) and Block documentation.

Removing inappropriate uses of add_entry

Removing more indirections via .entries

More cleaning up with use of the updated Block API, and adding a keys() method

Various fixes to ISAWIG output (and conversion to use new tuple-indexing on mixing matrices and single-value accessing on ALPHA).

Reordering TODO

Now targetting version 2.0.0 since changes have been more sweeping than envisaged

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