Density in density.txt is taken at 1 bar, not at 1.01325 bar #654

Describe the bug
All the density values listed in src/pandapipes/properties/<gas_name>/density.txt are stored at 1 bar (see example file: link). The formula that converts gas density at normal conditions, uses 1.01325 bar (NORMAL_PRESSURE) in its denominator. This inconcistency arises because the density from fluid.get_denstity() is returned at 1 bar. As a result, this leads to a 1.3% error in converting gas density from normal conditions.

Expected behavior
There should be consistency in normal conditions. I guess, the proper way to achieve this is to store gases' densities at 1.01325 bar

Python environment:

• pandapipes version: current HEAD on develop branch commit 1cd36e9