• Update package metadata
• Add Github Actions CI

Remove tests dependent on non-public implementations

map.market.test.R currently depends on the implementation
of the grobs in the grid package. Changes in this implementation
have caused failures in this test case. Removing until we create
a test not dependent on this implementation.

• API change where exposure and calcCandidates now explicitly
  return character vector instead of either factors or characters
  based on stringsAsFactors.

• Replace all global symbols in portfolio.R, portfolioBasic.R,
  and tradelist.R with explicit data.frame column lookups. Mostly,
  replaced usage of subset.

• Update map.market regression test to handle changes to grid
  in R 4.0.0. Thanks to Paul Murrell and Thomas Lin Pedersen
  for suggestions on the fix.