Make # Charge_Style a required keyword

[emarinri]

Is your feature request related to a problem? Please describe.

Users have requested that the keyword # Charge_Style is required instead of optional. Currently, if this keyword is not found, the electrostatic style is set to none and thus no electrostatic interactions are required.

Describe the solution you'd like

Make it a required keyword or set ewald as a default. For the first option, the user must decide if electrostatic computation is relevant for the problem. An alternative solution is to make ewald as a default and always compute electrostatic interactions. It seems to me that both options are safe. The second would involve a higher computational for uncharged systems if the keyword is not specified.

Additional context

mosdef_cassandra uses ewald as a default.