adds mat file containing string to BpodUserPath in Bpod home directory

[torbenott]

Enables custom user folder for Data, Protocol, Settings Files, and Calibration Files. Related to #2 without button (seems to much right now) and #6 @fitzsturgill .

• in default mode, starting Bpod creates a mat (now txt, see below) file in Bpod home directory containing a single string storing the default user location (HOME/BpodUser)
• this txt file can be edited to an arbitrary directory which will be used at next Bpod startup
• Warning: Calibration Files and Settings Files will be copied from the Bpod home directory to new location ("resets" settings/calibrations). Old locations will not be deleted. Data and Protocols have to be copied manually, if wished (as before).
• only tested in Win10

Potential issue: There is a new custom file in Bpod directory. But I think there has to be one, could also be contained in a settings file, but then this setting file should remain in Bpod home directory. The current proposal is a simple fix which can be easily changed in the future if sanworks/Bpod changes the way it stores files and is keeping the bpod changes to a minium.

[torbenott]

following @tsgouvea suggestion, I changed mat file to txt file, which is more easily editable #0b27cf3

[tsgouvea]

As there seem to be no sticking points, I'll merge it.