Reduce number of imports

[ethanplunkett]

The package check is adding a note that there are two many packages in imports.

To drop:

• progress - makes progress bar and is only used once

Move to suggests

These aren't widely used and requireNamespace() can be called to check for them before using them.

• rnaturalearthdata
• rhdf5 ? Not needed if getting models from collections.
• viridisLite - only used once. can store output and link to from help instead.
• gifski - Dave wanted this in imports so that it would always be installed otherwise gganimate writes a file for each frame. See strengthen dependencies #102. BirdFlowR never calls it directly. Personally, I think gganimate should import and use it as the default.

Consolidate?

• Matrix and SparseArray are slightly redundant. Matrix is used for sparse matrices SparseArray adds multi dimensional support. In theory we could maybe use SparseArray for both but it doesn't seem to support as many sub setting options.

Moving some functions into BirdFlowPipeline might alleviate some of the issues here. Moving the natural earth functions into their own package might also help. See #156.