WSL support

Changes to make fslpy work transparently when FSL is installed on Windows in the Windows Subsystem for Linux. The only change required by the user should be to specify FSLDIR using the WSL 2.0 network share method of referring to folders in WSL, for example:

FSLDIR=\\wsl$\Ubuntu-18.04\usr\local\fsl

This is a valid Windows path which enables FSL data and files to be accessed relative to FSLDIR as they would natively. Running wrappers for FSL commands has to identify that a WSL installation is being used, preprend some additional arguments to the command being run, and perform some path translation between the two. This is implemented as a separate case in fsl.utils.run.