The previous way of doing stuff was not allowing to load files from
multiple (sub)directories. This commits allows the user to specify
files from various directories as a string array.

BREAKING: This commit changes the input arguments for **xf2mat** and
**formatairfoilcoord**.

The two functions now return one single matrix with both coordinates
instead of separate vectors.

Nacaairfoil can also return the data in either Selig or Lednicer
formats.

This expand the scope of the function to convert any entry format into
an other output format.

Signed-off-by: Thomas Lambert <t.lambert@uliege.be>

Signed-off-by: Thomas Lambert <t.lambert@uliege.be>

Signed-off-by: Thomas Lambert <t.lambert@uliege.be>

If extended polars (_i.e._ polars given for [-180,180] deg) are given
as input, only look for values in the expected range of the original
polar.
- Stall should be in [0, 30] deg
- Zerolift should be in [-30, 30] deg
- Linear range for Cl should be in [-30, 30] deg

TODO:
- Fix and improve tests