Add class representing units.

- add new class in module fields
- add printout method.
- create unit test of new class
Rename files for represantation SI units.
Rename class for representation SI units.
Optimization of unit SI output.

- use format readable by LaTeX.
- optimization of methods what set exponents.
Add flag of undefined UnitSI object.
Modification of UnitSI class.

- add multiplicative operator
- add definitions of frequently used derived units
- add units test of multiplicative operator and derived units
Use of UnitSI in fields.
Enhance documentation of unit_si
Modify of unit_si.

- add operator of proportion
- add static definition of dimensionless unit
- optimization of class (rename print method)
- completion of unit tests
Fix of unit declarations.
Completion of unit definitions.
Use unit_si in documentation.

- add formatted strings of units to field_set methods generated
documentation.
Fix using UnitSI in unit tests.
Fix eq_data unit test.
Completion of unit definitions.
Fix definitions of derived units.

- use static methods for definitions
Use parameter of dimension in unit_si.

- add new unit represented m^d where d is dimension of region
- change format method
- use new unit in Fields
- add new unit to unit test
Fix dolar escaping problem in LaTeX documentation.
Merge branch '1.8.0_master' into DF_units
Fix dolar escaping problem.