Raise an error if ice density is calculated as negative (#8)

* raises an error if ice density is negative (for example due to porosity > 1) and fixed porosity documentation from percent to fractional

smrt/inputs/make_medium.py

@@ -234,7 +234,7 @@ def make_ice_column(ice_type,
     :param salinity: salinity of ice/water in kg/kg (see PSU constant in smrt module). Default is 0. If neither salinity nor brine_volume_fraction are given, the ice column is considered to consist of fresh water ice.
     :param brine_volume_fraction: brine / liquid water fraction in sea ice, optional parameter, if not given brine volume fraction is calculated from temperature and salinity in ~.smrt.permittivity.brine_volume_fraction
     :param density: density of ice layer in kg m :sup:`-3`
-    :param porosity: porosity of ice layer (in %). Default is 0. 
+    :param porosity: porosity of ice layer (0 - 1). Default is 0.
     :param add_water_substrate: Adds a substrate made of water below the ice column.
     Possible arguments are True (default) or False. If True looks for ice_type to determine if a saline or fresh water layer is added and/or uses the
     optional arguments 'water_temperature', 'water_salinity' of the water substrate.
@@ -479,6 +479,10 @@ def bulk_ice_density(temperature, salinity, porosity):
 
     # Density of mixture:
     rho = (1. - porosity) * (rho_ice * F1 / (F1 - rho_ice * salinity * PSU**-1 * F2)) * 1e3  # in kg/m3 (eq. 15, C&W, 1983)
+
+    if rho < 0:
+        raise SMRTError("Ice density may not be negative.")
+
     return rho
 
 
@@ -599,4 +603,4 @@ def compute_thickness_from_z(z):
     else:
         raise SMRTError("The z argument is not sorted")
 
-    return np.diff(z)
+    return np.diff(z)