Fix emphasis vs definitions in career and cass_koopmans_1

Changes per #721:
- career (career.md)
- job (career.md)
- aggregation theory (cass_koopmans_1.md)

All terms changed from italic to bold as they are definitions per style guide.

Author: mmcky

lectures/career.md

@@ -66,8 +66,8 @@ from matplotlib import cm
 
 In what follows we distinguish between a career and a job, where
 
-* a *career* is understood to be a general field encompassing many possible jobs, and
-* a *job*  is understood to be a position with a particular firm
+* a **career** is understood to be a general field encompassing many possible jobs, and
+* a **job**  is understood to be a position with a particular firm
 
 For workers, wages can be decomposed into the contribution of job and career
 

lectures/cass_koopmans_1.md

@@ -143,7 +143,7 @@ $$
 c(\omega) = c = C.
 $$
 
-This line of argument indicates the special *aggregation theory* that lies beneath outcomes in which a representative consumer
+This line of argument indicates the special **aggregation theory** that lies beneath outcomes in which a representative consumer
 consumes amount $C$.
 
 It appears often in aggregate economics.