Reparametrize lognormals for ecospold2 imports

[Loisel]

By the definition of the ecospold2 format, the mean of a lognormal distribution of an exchange is not equal to the amount specified for that exchange, see issue brightway-lca/brightway2-calc#76.

This PR adds a strategy which sets loc for lognormal distributions on imported databases such that the expected value (mean) of the resulting distribution is equal to the (static) amount specified for that exchange. More precisely,

$$ \mu = \log a - \frac {\sigma^2} 2 $$

where $a$ is the (static) amount and $\sigma$ is scale, i.e., the standard deviation of the underlying normal distribution.

The strategy is not applied by default but only if a specific flag (reparametrize_lognormals=True) is provided when initializing an instance of SingleOutputEcospold2Importer.