Fix definition of constant angstrom_star
Change the base unit of angstrom_star from 'dimensionless' to 'meter'.
NOTE: this bug did not exist in Pint v0.24.4 and appears to
have been introduced by PR https://github.com/hgrecco/pint/pull/2049.
Reference
- (p. 50) https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=958143