Use phases, splitting each pipeline into separate makefiles

[elmeraa-usgs]

Overview

Following issue #10, I split pipeline targets into the phases 1_fetch.R, 2_process.R, and 3_visualize.R and updated _targets.R to reflect changes by adding sourcing of new makefiles

Summary

• Addition of the three new makefiles listed above. I added the appropriate function sourcing to the associated makefile.
• Updated canonical _targets.R makefile by removing the targets and adding the 1_fetch.R, 2_process.R, and 3_visualize.R sourcing.

[github-learning-lab[bot]]

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