instructions for creating mfd are out of date

[chrisdicaprio]

The docs/usage.md contains the following instructions for creating an MFD from an inversion solution:

mfd = filtered_rates_df.groupby(
    pd.cut(
        filtered_rates_df.Magnitude,
        bins=bins,
    ))["rate_weighted_mean"].sum(numeric_only=False)

The column "rate_weighed_mean" is no longer used. Users should instead be directed to the mfd function already included in solvis:

from solvis.solvis import mfd_hist
mfd = mfd_hist(filtered_rates_df)

The mfd function should be hoisted to the top level package