feat: add concrete class

[mortenengen]

Continuation from #5.

First attempt to refactor concrete:

• one abstract class and then subclasses for each design code
• new implementation on how attributes are used: by default they assume a None value. If overwritten by user, will use the new values
• added optional description/name for materials and concrete. Default behaviors for names are defined
• created a factory function that creates the proper type of concrete based on the code wanted (the active global one or another specified by user)

Example usage:

import structuralcodes
from structuralcodes.material.concrete import create_concrete

# We can also create a specific class of material if needed
from structuralcodes.material.concrete import ConcreteMC2010

structuralcodes.set_design_code('mc2010')

# Create a concrete material with the active design code
c25 = create_concrete(25)

print('fcm = ', c25.fcm)
print('fctm = ', c25.fctm)
print('fctkmin = ', c25.fctkmin)

# We can override a quantity (useful for existing materials probably)
c25.fctm = 2.0
print('updated material\n', 'fctm = ', c25.fctm)

# If we want we can reset it to default values computed by MC2010
c25.fck = 25
print('restored material\n', 'fctm = ', c25.fctm)

# We can also create a material with a given code (not the used active one)
c25 = create_concrete(25, design_code='mc2010')
print('fcm = ', c25.fcm)

# We can also create a specific object of the specific concrete class
c30mc = ConcreteMC2010(30)
print('fcm of the new specific instance: ', c30mc.fcm)

# A new existing concrete object
c30ex = ConcreteMC2010(30, existing=True)
# For now this commands raises a not implemented exception

TO BE DONE STILL:

• split the class in more files (for now it is all in a single concrete.py file): one file for abstract class and factory function + one file for each concrete implementation for the different codes
• add tests
• some more value checking in the setters of properties in concrete
• improve docstrings (sphinx format is ok?)