Update Excel dependency

[dan-knight]

gData no longer supports Excel format as of v3.x.x. The readxl package appears to be the new standard. Though it is a part of Tidyverse, this change imports it alone. "readxl has no external dependencies", so importing the package separately from Tidyverse will be extremely minimal.

Some extra checks were required for the output to be consistent with previous gData implementation. But, this should now integrate seamlessly with any existing package functionality and external code which uses the affected functions.

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[dan-knight]

Closing this as we're taking a different approach. readxl depends on tibble from Tidyverse. Lab policy is to avoid Tidyverse dependencies.