Create working version of function calculate_mana_cost

Created a working version of the calculate_mana_cost function that
calculates the mana cost of a skill based on the linked support gems.
The resulted value is rounded up so it actually is an integer.
Updated calculate.py and functionslist

Added code to the following functions in calculate.py:
- calculate_mana_cost
- calculate_damage_per_second
- calculate_crit_chance

Also added calls to the functions to test the in-game values. This will
be added to unittests later. The unittests will be updated on a regular
basis with the actual values to be able to compare them to the actual
values.