modified requirements.txt

modified setup.py. sxstools version updated

Update README.md

Update test_nrsxs.ipynb

modified the nrsxs.py and setup.py

Update README.md

modified nrsxs.py

added a support for f_low correction and modified test_nrsxs.ipynb

added a support for estimating waveforms at a particular reference freqeuncy

corrected the default extrapolation

modified nrsxs.py test_nrsxs.ipynb

modified test_nrsxs.ipynb

modified readme

modified setup.py and nrsxs.py

modified setup.py