Fluctuating charges MM

Added

• An implementation of the fluctuating charge (FQ) molecular mechanics (MM) model.
• A new API initialization function pcmsolver_new_read_host. This allows
  creating an uninitialized pcmsolver_context_t object: the initialization is
  deferred to a later point and orchestrated by the host program. The input
  options are then set via the other API functions:
  • pcmsolver_set_bool_option
  • pcmsolver_set_int_option
  • pcmsolver_set_double_option
  • pcmsolver_set_string_option
    A call to pcmsolver_refresh will trigger initialization of all internal
    objects managed through pcmsolver_context_t. Use this function when you want
    to manage input parsing on the host side, rather than use a dedicated input file
    for PCMSolver.
• A pcmsolver_fill_pcminput function in the Fortran interface. This function
  simplifies the creation of the PCMInput object from Fortran.
• A PCMSOLVER_WARNING macro to emit warning to users.

Changed

• Use GitHub Actions instead of Travis for continuous integration.
• Remove integration with Danger in CI.
• A C++11-compliant compilers is required to compile the library.
• The Fortran interface has been refactored to hide the transformation of
  strings from Fortran to C from the host program.
• The check on the symmetric positive definiteness of the S matrix triggers a
  warning, not an error.
• We switched back to using LU decomposition for CPCM.
• For IEF, we switched to LU decomposition, instead of partially pivoted LU.
• UFF radii are scaled by 1.1 by default.

Fixed

• The printout of sphere centers and radii always uses Angstrom.