Add PMrel contrail capability

[liammegill]

In AirClim 2.1, a reduction in PM emissions also results in a reduction in contrail cirrus RF, according to simulations by Burkhardt et al. (2018). The fit of these simulations was found to be:

$$x = 0.92 * \arctan(1.902 \cdot PM_x ^ {0.74} )$$

for $PM_x$ > 3.3 %. This can also be added to oac.cont.

[liammegill]

A simple PMrel factor has been implemented into oac.cont with commit d37c6cf. This counts for the full emission inventory, so it is not yet possible to change only part of the input emission inventory. This will be addressed in #16.