density profile hotfix

[mklauser]

Hotfix for issue #82

[coveralls]

Coverage remained the same when pulling f64d02b on mklauser:master into 25d4568 on tardis-sn:master.

[ssim]

This mostly looks good. I think we want to change the parameters used in the "exponential" case, however (see 1 below) and need to make use of time_0 for both the exponential and power-law cases (see 2 below).

(1) If I understand correctly, the exponential profile is being parametrized as:

rho = const_a[parameter] * rho_0[parameter] * exp( -velocity / velocity[shell0])

this is not very flexible and it enforces that the e-folding rate is related to the location of the innermost shell velocity. I would suggest we use

rho = rho_0[parameter] * exp(-velocity / vel_0[parameter])

…so the user specifies two things - the e-folding velocity parameter (vel_0) and the normalisation (rho_0 -> which is interpreted as the density for the profile as velocity->0).

(2) in both the density profiles (exponential and power-law) we are introducing a time_0, which is good I think. However, I don't see where this is being used - is there an equivalent to the line

densities = calculate_density_after_time(densities, time_0, time_explosion)

from the branch_w7 profile missing, or is this being done elsewhere? The lines about reading this parameter (the messages being printed) should be clarified for the density and exponential cases (don't make reference to the branchw7 case).

[wkerzendorf]

This looks good, but it seems intertwined with PR #80. So before we implement this let's get #80 done and then rebase this PR on the then new master (if you have questions about rebasing just ping me).