Change the definition of pc1, pc2 and pc3

These values are no longer defined as a string plus a percent sign,
these are now defined as empty variables so the python code can add
whatever is needed.

The python code now does something like this for pc1: PC1 (25 %) when
writting the javascript code, the code is also now capable of handling
custom axes names.

Tests were updated accordingly.
Update script to correctly label axes names
Merge pull request #27 from ElDeveloper/issue_14

Issue 14 -- axes names