v1.9.0

Release cpymad v1.9.0

API changes:

- ``Table.selected_rows()`` now actually returns the *indices* of the selected
  elements as documented, rather than returning a boolean mask.
- ``Madx.eval()`` does no automatic syntax checking anymore. This is a minor
  performance improvement and is more consistent with ``Madx.input()`` which
  doesn't check the syntax either. Expressions can still be checked manually
  using ``cpymad.util.check_expression``

New features:

- (#90) Add comparison operators for ArrayAttribute (see #89)
- (#94) Add keyword argument ``Table.dframe(index=..)`` to allow specifying
  a column or sequence as the DataFrame index rather than using the default
  (``row_names()``). This is essential when accessing a table after having
  executed a ``USE`` statement (see #93).
- (#97) Add basic support for unexpanded nested sequences by returning
  them as elements of type ``Sequence`` from ``Sequence.elements`` (see #76)
- Add keyword argument ``Madx(prompt=...)`` as a shortcut for the most common
  ``CommandLog`` use case
- (#99) Add method ``Table.column()`` to retrieve specified rows in a specific
  column.
- (#99) Add ``rows`` and/or ``columns`` arguments to several ``Table`` methods
  to allow querying only specific columns or rows from the MAD-X process
- (#99) Add method ``Table.selection()`` that returns a new ``Table`` object
  which exposes only those rows/columns marked by a previous ``SELECT`` statement
  (see #98)
- Fix ``Madx.eval()`` to handle function calls, e.g.: ``sin(...)`` or ``table(...)``

Bug fixes:

- (#95) Fix ``KeyError`` when accessing tables after ``CLEAR`` (see #57)
- (#99) Fix requesting a subset of table rows using using a *numpy array*
  of indices
- Fix expression syntax checker to not reject function calls anymore. Rewrite
  the checker to use a LL(1) parser generator.

Documentation:

- Add links to external documentation using intersphinx
- Use automodapi to create module and class summary pages
- Use type hints from function annotations in documentation
- Mark more objects for inclusion in the documentation
- Add "Edit on GitHub" link to directly edit documentation
- Add many function annotations
- Generate class inheritance diagrams using graphviz
- Document problem with ``Table.row_names()`` after ``USE`` (#93)

Tests/CI:

- Move python 3.5 deprecation warning to package level
- Create GitHub releases for tags automatically (required for zenodo DOIs)
- Setup sequence definitions individually and explicitly in each test
- Add module for regression tests for all future bugfixes
- Put transfer map tests in their own module
- Port tests from unittest to pytest for simplicity
- Mark flaky tests for expected failure on macOS and windows