[BUG] json (pas-file) float precision

[dbrakenhoff]

Describe the bug
The float precision when calling DataFrame.to_json() is 10 by default (max 15) and stores numbers in decimal format when it would be much more logical to store most values in scientific format. This means that after loading a model from a pas-file, the float precision is affected which can lead to unexpected results.

To Reproduce
Get some model with a very small parameter value somewhere in ml.parameters or ml.fit.pcov and then store model as pas file:

ml.to_file(f"{ml.name}.pas")
ml2 = ps.io.load(f"{ml.name}.pas")
# try one of these, depending on which DataFrame contains the small float value
ml.fit.pcov.equals(ml2.fit.pcov)
ml.parameters.equals(ml2.parameters)

Expected behavior
Store values with higher precision and scientific notation in pas files.

[dbrakenhoff]

Closed by #354