Remove low_memory=False from read_dataframe

When running:
```
pd.read_csv(DF_PATHS['spmrl'], compression='gzip')
```
You get the following warning:
```
DtypeWarning: Columns (21) have mixed types. Specify dtype option on import or set low_memory=False.
  interactivity=interactivity, compiler=compiler, result=result)
```

Find the cause for this, fix it and remove `low_memory=False` from `read_dataframe`