Padding in MultiChannelPT and a central function for duration comparability test

[terrorfisch]

As a first step I will factor out _as_expression and add a initial_value and final_value to PulseTemplate

Factored out central duration compability test in #590

TODO:

• test
• configure tolerance as user (how?)

[coveralls]

Pull Request Test Coverage Report for Build 2187

• 238 of 270 (88.15%) changed or added relevant lines in 11 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 84.27%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qupulse/pulses/arithmetic_pulse_template.py	9	10	90.0%
qupulse/utils/numeric.py	13	14	92.86%
qupulse/_program/waveforms.py	86	88	97.73%
qupulse/pulses/function_pulse_template.py	1	3	33.33%
qupulse/pulses/mapping_pulse_template.py	1	4	25.0%
qupulse/pulses/table_pulse_template.py	24	27	88.89%
qupulse/utils/sympy.py	4	8	50.0%
qupulse/pulses/multi_channel_pulse_template.py	67	83	80.72%

Files with Coverage Reduction	New Missed Lines	%
qupulse/pulses/table_pulse_template.py	1	96.56%
qupulse/utils/types.py	1	87.57%

Totals
Change from base Build 2175:	-0.1%
Covered Lines:	6461
Relevant Lines:	7522

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💛 - Coveralls