Read multi-column csvs in PhyIO

[Kleinjohann]

PhyIO automatically loads csv files in the data folder, but assumes that they only have two columns. This causes an error e.g. when loading a SpikeInterface folder with quality metrics which are saved in a single multi-column 'metrics.csv' file.

I extended the csv loading routine to allow for multiple columns, and while doing that I noticed that negative floats currently cause an error since they are not covered by the regular expression that is being used to detect floats, so I fixed that, too. Additionally, I added tests covering the multi-column and negative float cases.