flexible data path function points to r2dii.stress.test.data by default

[jacobvjk]

The new repo needs to allow the user to read in data from directories outside of the repository

as such the data path function needs to be overhauled.
It should point to the r2dii.stress.test.data repo by default, assuming that repo is in the same directory as r2dii.climate.stress.test
The user should be able to point the data path to any other place if necessary

for reference: https://dev.azure.com/2DegreesInvesting/2DegreesInvesting/_workitems/edit/503