Include the conversion factor in a Unit (#78)

* add `factor` attribute in `Unit`

* enable mathematical functions support `factor` in `Unit`

* add molecular dynamics units using `factor` of `Unit`

* `Unit.factorless` is always correct

* add more constant units

* fix test bugs

* fix test bugs

brainunit/__init__.py

@@ -16,21 +16,30 @@
 __version__ = "0.0.3"
 
 from . import _matplotlib_compat
-from . import lax
-from . import math
-from . import linalg
 from . import autograd
+from . import constants
 from . import fft
+from . import lax
+from . import linalg
+from . import math
 from ._base import *
 from ._base import __all__ as _base_all
 from ._celsius import *
 from ._celsius import __all__ as _celsius_all
 from ._unit_common import *
 from ._unit_common import __all__ as _common_all
 from ._unit_constants import *
-from ._unit_constants import __all__ as _constants_all
 from ._unit_shortcuts import *
 from ._unit_shortcuts import __all__ as _std_units_all
+from .constants import *
+from .constants import __all__ as _constants_all
 
-__all__ = ['math', 'linalg', 'autograd', 'fft'] + _common_all + _std_units_all + _constants_all + _base_all + _celsius_all
+__all__ = (
+    ['math', 'linalg', 'autograd', 'fft', 'constants'] +
+    _common_all +
+    _std_units_all +
+    _constants_all +
+    _base_all +
+    _celsius_all
+)
 del _common_all, _std_units_all, _constants_all, _base_all, _celsius_all, _matplotlib_compat