## e_tot calculation for storage

### Summary

The e_tot calculation is defined in EnergyUnits as :

`{0}_e_tot == time.DT * lpSum({0}_p[t]for t in time.I`

while p[t] in storage units is calculated as :

`{0}_p[t] == {0}_pc[t] - {0}_pd[t]`

### Steps to reproduce

Opening the storage design example and printing e_tot
`print(STORAGE.e_tot)`

###
*bug* behavior?

What is the current The previous command gives a wrong result for e_tot but the right result for the difference between e_ch and e_disch that can be obtained as follow:

`print("sum pcharge", sum(STORAGE.pc.value.values())) print("sum pdischarge", sum(STORAGE.pd.value.values()))`

###
*correct* behavior?

What is the expected e_ch and e_disch should be created and easily accessible, and e_tot also accessible but indicated as the difference between e_ch and e_disch