dplyr check onLoad via search() path order
gui fixes

- fixes #164. Expanded the error message to name the layer and spatial field without associated spatial shapes for mapping. Should no longer cause the screen to gray, just a red message saying:

    Error: Sorry, the Map view is unavailable for the selected layer (fis_b_bmsy) because the field fao_id does not have associated spatial shapes available for mapping. Please choose a different view (Histogram or Table) or a different layer.

- fixes #95. Handled this odd single zero value condition by using an arbitrary range.

    rng = c(-1, 1) * 0.001

- fixes #165. Recently removed the scenario.R file since standardized expected names of files and folders within. Updated code to not require this file but still set vars for running rest of Calculate.
Merge pull request #166 from bbest/master

gui fixes and smarter dplyr onLoad check