Fix plots: Make load_over_time Function Dynamic

[tichmangono]

Plotting with the load_over_time function was giving an error due to hardcoded indexes to select columns to be plotted. I suggest changes to make this part dynamic so it will plot only the numerical columns and only when there are numerical columns. Also wrote a comprehensive set of tests for the function with different test cases.

Problem code to reproduce error:

import gridstatus
import plotly.express as px
import pandas as pd

iso = gridstatus.PJM()
df = iso.get_load(date="today")

gridstatus.viz.load_over_time(df, iso="PJM")

The error seen:

`ValueError: Plotly Express cannot process wide-form data with columns of different type.`

Solution
Used pandas column datatype filters to identify numerical columns, then pass these to be plotted as the dependent variables (y)