pipeline error in eGon2035.py: append-function does't match with pandas version

[lenzim97]

• In eGon2035.py (line 324) the append- function is called, which leads to an pandas-error cause of pandas-version.

Current workaround:
old code: grouped_capacities = grouped_capacities.append(df_conv_2035)
fixed code: rouped_capacities = pd.concat([grouped_capacities, df_conv_2035], ignore_index=True)

• same problem for the task insert-chp-egon2035:
  in function match_nep.py append-function is called either multiple times