adding valueError to raise when the code value is out of range

[sjpark608]

Closes #290

• raises valueError when the resulting code of enable_dac_gain() code handler exceeds the maximum allowable code value
• revised unittests

[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
adc	87%	86%	✔
calibration	86%	89%	✔
dac	100%	97%	✔
digital_input	100%	100%	✔
digital_output	100%	100%	✔
gpio	98%	94%	✔
led	0%	0%	❌
peripherals	88%	100%	✔
reg_helper	97%	85%	✔
relay	100%	100%	✔
tc	98%	98%	✔
utilities	100%	100%	✔
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