need to manually import orjson library to use nlinfit

[cchilkun]

while using pycse nlinfit, found an import error with the orjson library -- a simple pip install orjson fixed this, but wanted to bring this to your attention

[jkitchin]

that seems weird. orjson is listed in the requirements.txt file, and only used in hashcache.py. I am not sure why it would do that.