massXpert2 is the successor of massXpert. massXpert was put
in maintenance mode at version 7.0.0 (last Debian package 7.0.0-2).

massXpert2 is a deep refactoring of massXpert involving the following
changes:

- modularization of all the gui and nongui code that might be useful
  also for the mineXpert2 project in two private static libraries
  libmass and libmassgui.

- libmass and libmassgui are in the source tree in the form of git
  submodules. Their code is located at:

  - https://gitlab.com/msxpertsuite/libmass
  - https://gitlab.com/msxpertsuite/libmassgui

- full refactoring of the chemical element modelling, that is, no more
  based on the Isotope/Atom/AtomCount classes but on a new Isotope class
  that mimicks the data structure of the IsoSpec++ library data.  That
  Isotope class describes an isotope. The full set of chemical element
  data is stored in IsotopicData, a class that contains a vector of all
  the Isotopes instances of a given chemical-element-based environment
  (a polymer chemistry definition has one IsotopicData member). The
  IsotopicData class has all the functions required to handle the large
  std::vector<Isotope *>, allowing amongst other things to:

  - gather all the isotopes by the same symbol;
  - compute an average mass for a given set of isotopes or for all the
    isotopes by a given symbol;
  - find the monoisotopic mass for a given symbol or for a given set of
    isotopes;
  - ... All these functions permit to perform on the fly all the
    calculations that were coded in Isotope/Atom/AtomCount and
    QList<Atom> in massXpert.

 In addition to the std::vector<Isotope *>, the IsotopicData class has
 two maps relating the each symbol in the data with its monoisotopic
 mass and with its average mass:

 - std::map<QString, double> relating symbol and mono mass;
 - std::map<QString, double> relating symbol and avg mass;

 A large set of functions allow inserting, appending and removing
 isotope instances of any given symbol from the vector with automatic
 update of the average and mono mass for that symbol in the
 corresponding map. Lookup of the masses of any given symbol is
 extremely fast with these two maps.

- The modifications above (in libmass) are accompanied by a full set of
  GUI modifications in:

  - libmassgui, for all the new GUI'nterfaces that might be useful in
    mineXpert2 also;
  - src/gui, for the new version of GUI'nterfaces that are specifically
    designed for the new massXpert2.

The present commit is the first commit (all precedent commits were
squashed into this one) that features a set of functionalities on par
with those found in massXpert. Thorough testing will certainly highlight
some glitches, but this commit has been tested by comparing with
massXpert and is seemingly pretty functional with no differences
visibles with respect to massXpert.

For the record, massXpert is located at all these repositories:

- https://gitlab.com/msxpertsuite/massxpert
- https://forgemia.inra.fr/pappso/massxpert
- https://salsa.debian.org/debichem-team/massxpert