Lots of functions need unitary tests

I think lots of functions are not so obvious to understand and could easily lead to bugs.
Writing unitary test for them within unittest framework would be a great idea for:

- be sure that the code is bulletproof.
- make refactoring and maintenance much easier
- help to understand what the code does.

Example with this:

```
class Steel(SteelBase):

def __init__(self, name, fy, fu, eu, E, v, p):
super(Steel, self).__init__(name, fy, fu, eu, E, v, p)
self.data = self._generate_data()

def _generate_data(self):
data_section = []
line = """*Material, name={}
*Density
{},
*Elastic
{}, {}
*Plastic""".format(self.name, self.p, self.E['E'], self.v['v'])
data_section.append(line)

for i, j in zip(self.compression['f'], self.compression['e']):
line = """{}, {}""".format(abs(i), abs(j))
data_section.append(line)
return '\n'.join(data_section)
```
If you take a month of holiday and I am in charge of the maintenance without knowing Abaqus, your part of the code, a small test case for this function will give me all the cards in my hand for fixing a bug. I will know exactly what is inside each variable and what to expect from the function.