Assure correct dtypes on attribute columns

[MariusWirtz]

In get_elements_dataframe function

Fixes #1069

[Kevin-Dekker]

Before installing this branch of tm1py, string attributes come back as floats:

After installing this branch of tm1py, string attributes come back as string:

After installing this branch of tm1py, numeric attributes remain numeric:

After installing this branch of tm1py, numeric attributes remain numeric, string attributes come back as string, when retrieving both:

Code to reproduce (create a stringattribute like 'StringAttr' and fill it with only numbers):

python df_from_use_blob_true = tm1.elements.get_elements_dataframe( dimension_name='account1', hierarchy_name='account1', elements=['Price', 'Units'], skip_weights=True, skip_parents=True, attributes=['StringAttr', ], use_blob=True)