LE trend working

- need to clean up comments and commit layers to layers_global

- fixed trend to use proper slope:

```R
Here's an example using filter(metric=='wages' & country=='AIA' & sector=='ph').

px = c(1995, 2000)
py = c(9729.496824548931, 9729.489868975525)

# OLD: incorrect
slope = (py[2] - py[1]) / py[1]

# NEW: correct
slope = (py[2] - py[1]) / (px[2] - px[1])
```

In practice, all the original code attached (and with this fix of correctly using the annual correlation coefficient from the linear model) is efficiently coded with dplyr like so:

```R
# trend per metric-country-sector
mcs = mcsy %>%
  filter(!is.na(value)) %>%
  group_by(metric, cntry_key, sector) %>%
  do(mdl = lm(value ~ year, data=.)) %>%
  summarize(
    metric    = metric,
    cntry_key = cntry_key,
    sector    = sector,
    trend     = max(-1, min(1, coef(mdl)[['year']] * 5)))
```

fixes ohi-science/ohicore#128