Hmf

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**Changes**

- ``camb`` is no longer installed by default. If you want to install it along with ``hmf``
(as well as other useful utilities) install with ``pip install hmf[all]``.

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**Bugfixes**

- Faster and more reliable halofit! `PR 106 <https://github.com/steven-murray/hmf/pull/106>`_

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**Features**

- Option to extrapolate high-k for CAMB transfer using EH. This gives a fast and
reasonably consistent high-k transfer function. If not extrapolating with EH (which
is still the default) you can now set ``transfer_params={'kmax':1e3}`` to get better
low-mass/small-scale accuracy. By default, kmax is the same as the default from
CAMB, which is about 1.4 h/Mpc. Above this value, the transfer function is by default
linearly extrapolated in log-log space.
- Components now automatically keep track of models, and the base component class
tracks Component types. This also means that user-defined models can be input via
strings into a framework (useful for running from CLI).

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**Features**

- Updated (working) CLI: use ``hmf run`` to produce mass functions on the command-line!

**Internals**

- Deprecated astropy_to_colossus function.

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**Bugfixes**

- Consistently allow string "None" for some parameters.

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**Notes**

New versions of CAMB (1.0.0+) seem to run significantly slower by default than previous
versions. This could just be a setting in CAMB that has changed its default. Be aware
of this.

**Features**

- Added new ``CambGrowth`` growth factor model, which computes the growth using CAMB. This is useful especially when
using w > -1, for which the other growth factor models are inadequate. Solves issue 19 raised by tijmen.
- Added new module ``mass_definitions`` which more robustly deals with various halo mass definitions, and also includes
ability to convert mass functions between different definitions.
- New set of tutorials: quickstart/first plot and a broad overview.
- Added shortcuts to ``halo_overdensity_mean`` and ``halo_overdensity_crit``
- Added ``normalized_filter`` to ``MassFunction`` class to make it easier to use a filter
with a pre-normalized power spectrum.
- Added ``.clone()`` method to all ``Framework`` classes, which gives ability to clone
the current instance (while also changing some parameters).

**Bugfixes**

- When using camb for the transfer function, some cosmologies would lead to a segfault (i.e. when Ob0 or Tcmb0 are not
set explicitly). This now raises a helpful error.
- Fixed bug in ordering of iterator in ``get_hmf``.

**Internals**

- Removed logging, which was redundant.
- Moved from nose to pytest
- Significant overhaul of package structure to more modularised form.
- Added flake8 checks and black formatting
- Moved away from git-flow as it is not easy to use with GitHub.