add pandas to requirements

I tried to update from a really old install and then got the error:

>pip install -e .
Obtaining file:///home/sm_lbarr/CODE/climix
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Preparing editable metadata (pyproject.toml) ... done
...
Building wheels for collected packages: climix
  Building editable for climix (pyproject.toml) ... done
...
Successfully installed bottleneck-1.3.7 climix-0.20.0.dev3+g6efeda6 heapdict-1.0.1 isodate-0.6.1 msgpack-1.0.7 psutil-5.9.6 sortedcontainers-2.4.0 tblib-3.0.0 tornado-6.3.3
>climix -h
Traceback (most recent call last):
  File "/home/sm_lbarr/.conda/envs/climix-devel/bin/climix", line 5, in <module>
    from climix.main import main
  File "/home/sm_lbarr/CODE/climix/climix/main.py", line 16, in <module>
    from .datahandling import prepare_input_data, save
  File "/home/sm_lbarr/CODE/climix/climix/datahandling.py", line 16, in <module>
    from .util import find_cube_differences
  File "/home/sm_lbarr/CODE/climix/climix/util/__init__.py", line 4, in <module>
    from .cube_diffs import find_cube_differences
  File "/home/sm_lbarr/CODE/climix/climix/util/cube_diffs.py", line 7, in <module>
    import pandas as pd
ModuleNotFoundError: No module named 'pandas'

A manual install of pandas did the trick.