constants: Update to 2018 CODATA values

The constants package contains CODATA internationally recommended values of the fundamental physical constants, provided as symbols for direct use within the R language. Optionally, the values with uncertainties and/or units are also provided if the errors, units and/or quantities packages are installed. The Committee on Data for Science and Technology (CODATA) is an interdisciplinary committee of the International Council for Science which periodically provides the internationally accepted set of values of the fundamental physical constants. This release contains the “2018 CODATA” version, published on May 2019 [E. Tiesinga, P. J. Mohr, D. B. Newell, and B. N. Taylor (2020) http://physics.nist.gov/constants].

This version contains some breaking changes that are necessary to streamline future updates and provide a stable symbol table:

• The codata table includes the absolute uncertainty instead of the relative one. Thus, the rel_uncertainty column has been dropped in favour of the new uncertainty. Also, columns have been slightly reordered.
• Symbol names for constants have changed. The old ones were hand-crafted and thus unmanageable. This release adopts the ASCII symbols defined by NIST in their webpage, except for those that collide with some base R function. In particular, there are two cases: c, the speed of light, has been renamed as c0; sigma, the Stefan-Boltzmann constant, has been renamed as sigma0.
• Constant types, or categories, (column codata$type) adopts the names defined by NIST in the webpage too. Some constants belong to more than one category (separated by comma); some others belong to no category (missing type).

There are some new features too:

• In addition to the codata data frame, this release includes codata.cor, a correlation matrix for all the constants.
• In addition to syms_with_errors and syms_with_units, there is a new list of symbols called syms_with_quantities (available if the optional quantities package is installed), which provides constant values with uncertainty and units.
• Experimental support for correlated values in syms_with_errors and syms_with_quantities is provided (disabled by default; see details in help(syms) for activation instructions).

See the README for some usage examples. For questions, suggestions or issues, please use the issue tracker.