fix: correct DataSet method return type declarations

Author: kmannislands

index.d.ts

@@ -37,62 +37,62 @@ declare module 'dicom-parser' {
     /**
      * Finds the element for tag and returns an unsigned int 16 if it exists and has data. Use this function for VR type US.
      */
-    uint16: (tag: string, index?: number) => number;
+    uint16: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and returns a signed int 16 if it exists and has data. Use this function for VR type SS.
      */
-    int16: (tag: string, index?: number) => number;
+    int16: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and returns an unsigned int 32 if it exists and has data. Use this function for VR type UL.
      */
-    uint32: (tag: string, index?: number) => number;
+    uint32: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and returns a signed int 32 if it exists and has data. Use this function for VR type SL.
      */
-    int32: (tag: string, index?: number) => number;
+    int32: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and returns a 32 bit floating point number if it exists and has data. Use this function for VR type FL.
      */
-    float: (tag: string, index?: number) => number;
+    float: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and returns a 64 bit floating point number if it exists and has data. Use this function for VR type FD.
      */
-    double: (tag: string, index?: number) => number;
+    double: (tag: string, index?: number) => number | undefined;
 
     /**
      * Returns the actual Value Multiplicity of an element - the number of values in a multi-valued element.
      */
-    numStringValues: (tag: string) => number;
+    numStringValues: (tag: string) => number | undefined;
 
     /**
      * Finds the element for tag and returns a string if it exists and has data. Use this function for VR types AE, CS, SH, and LO.
      */
-    string: (tag: string, index?: number) => string;
+    string: (tag: string, index?: number) => string | undefined;
 
     /**
      * Finds the element for tag and returns a string with the leading spaces preserved and trailing spaces removed if it exists and has data. Use this function for VR types UT, ST, and LT.
      */
-    text: (tag: string, index?: number) => string;
+    text: (tag: string, index?: number) => string | undefined;
 
     /**
      * Finds the element for tag and parses a string to a float if it exists and has data. Use this function for VR type DS.
      */
-    floatString: (tag: string, index?: number) => number;
+    floatString: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and parses a string to an integer if it exists and has data. Use this function for VR type IS.
      */
-    intString: (tag: string, index?: number) => number;
+    intString: (tag: string, index?: number) => number | undefined;
 
     /**
      * Finds the element for tag and parses an element tag according to the 'AT' VR definition if it exists and has data. Use this function for VR type AT.
      */
-    attributeTag: (tag: string) => string;
+    attributeTag: (tag: string) => string | undefined;
   }
 
   export interface ByteStream {