Add usage to README and add tests with private data and fix a few bugs

[je-cook]

• Adds some basic usage data to the README.
• Pulls over some more tests from bluemira
• Fixes 3 bugs
  1. old json eqdsks sometimes contain pnorm instead of psinorm
  2. fix eqdsk writing bug
  3. In the strict g-eqdsk standard qpsi must have the same length as nx.
     • This is now enforced in an eqdsk file only
     • A choice should be made about the fill value, if qpsi is a single value we fill with that, if it is multiple we fill with 0 to the length of nx, if qpsi is None we fill with 0

[oliverfunk]

Re. qpsi not being defined in an eqdsk file, or not being the correct length (same length as nx)

• For qpsi not being defined: it should be set to an array of 1's, length of nx
• For qpsi not being the correct length: we should raise an error.

[oliverfunk]

Few nits, otherwise all good!