EL1 Tweaks

[siwelwerd]

• Not sure Definition 5.1.9 is a definition--what are we defining here? Grow/Decay? Feels more like an observation as stated. If we really want this to be a definition, use <term>.
• Remark 5.1.14 What do we mean by "growing at a continuous rate" here? I think I'd say $2^x$ also grows at a continuous rate. I'm guessing we are alluding to continuous vs. discrete compounding here.
• Definition 5.1.15 isn't actually defining $e$.

I don't know how much we want to get into motivating $e$ here, or if that should be an appendix, or what. Active Prelude devotes a whole section to this. My general inclination is that nothing should be unmotivated, but this doesn't quite feel like the place. Maybe we could expand Activity 5.7.9 to continuous compounding to motivate the definition $e=\lim _{n \rightarrow \infty} \left(1+\frac{1}{n}\right)^n$ (if I remember right, this is actually the application that motivated Bernoulli to consider $e$ in the first place).

[kathypinzon]

@AbbyANoble @tdegeorge talk about this - I have added an aside to that see that activity.....

[siwelwerd]

Consensus from the discussion today was to expand Remark 5.1.14 a little, and then move 5.1.14 and 5.1.15 to the bottom. @tdegeorge is going to add an activity into 5.7 that really motivates $e$ (see #314).