Phydms

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* Altered ``setup.py`` to enable installation without ``numpy`` being pre-installed.

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* Added ``--gammabeta`` option to ``phydms``

* Added ``ExpCM`` with ``divpressure`` to ``simulate.py``.

* Updated citation

* Fixed bug in ``simulate.py`` that caused incorrect branch length scalings

* Change installation recommendations

* Fixed installation of ``cython`` extensions.

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* Added ``--opt_details`` argument to ``phydms``

* Tweak to likelihood optimization convergence tests

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* Minor tweaks to `phydmslib.simulate`

* Fixed rare bug in likelihood optimization that caused nothing to be optimized

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* Changes to reduce problems with convergence in ``--diffprefsbysite``

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* Added new simulation capabilities to `phydmslib.simulate`

* Added ``--initparams`` option to ``phydms``

* Modified ``--omegabysite`` fitting to reduce (hopefully eliminate) rare errors