Fix dependency issues

[SabineHaas]

There are several dependency issues when installing pvcompare which need to be looked into:

xarray 0.16.1 requires pandas>=0.25, but you'll have pandas 0.24.2 which is incompatible.
oemof 0.3.2 requires pyomo<5.7,>=4.4.0, but you'll have pyomo 5.7.1 which is incompatible.
greco-technologies 0.0.1 requires feedinlib==0.0.12, but you'll have feedinlib 0.1.0rc2 which is incompatible.
greco-technologies 0.0.1 requires xarray==0.15.0, but you'll have xarray 0.16.1 which is incompatible.

[SabineHaas]

oemof 0.3.2 requires pyomo<5.7,>=4.4.0, but you'll have pyomo 5.7.1 which is incompatible.

oemof 0.3.2 is not needed, find out in which package it is installed.

greco-technologies 0.0.1 requires feedinlib==0.0.12, but you'll have feedinlib 0.1.0rc2 which is incompatible.
greco-technologies 0.0.1 requires xarray==0.15.0, but you'll have xarray 0.16.1 which is incompatible.

See #81

[AnshuTrivedi]

I can fix this issue.
Can you assign me ?

[SabineHaas]

Thank you @AnshuTrivedi, I've already written issues in other projects that lead to the dependency issues.

open_FRED cli imports oemof 0.3.2 which is not needed, as far as I can see - I've asked to remove the requirement. open-fred/cli#16
I've further asked in demandlib to loosen the limit of pandas to >= 0.25. oemof/demandlib#27

The remaining dependency issues were solved in greco_technologies.

[SabineHaas]

Update: with the next demandlib release the xarray/pandas dependency issue will be solved (see PR)