add complexity measure as pure virtual Base method

[lubgr]

• can be used to decide whether the result of a transformation is worth storing in the cache (e.g. normalization) and maybe later on for a simplify() method
• pure virtual and const method complexity(), returning an unsignend in Base
• the implemented methods in Base-derived classes can e.g. return
  • Undefined 0
  • Numeric: integer 1, fraction 2, double 3
  • Constant 4
  • Symbol 5 (for both positive and non-positive)
  • Sum 5 + operands
  • Product 5 + operands
  • Power 5 + base + 2*exp
  • Function 6 + operands
• unit tests should all be in a new test/testcomplexity.cpp

[lubgr]

Resolved by f50f3db