Add function to import data file with alt MR
Fix bug and modify functions to include inset

- All plot_obs_pred functions taking multiple sites for input are
modified so that inset can be included.
- Previously R**2 was calculated using raw obs and pred values. Given
the magnitude some of these values span, it should be calculated on log
scale instead.
More fixes with respect to log-transformation in R**2

Calculation of R**2 is now consistent with the plot in scale, i.e., if
the data are log-transformed in creating the plot then they are logged
for R**2 as well. Before log-transformation, obs-pred pairs are removed
when at least one of them is zero.